The economics of Faerun's Inner Sea

Among the denizens of our forums, there are many people whose grasp of economic theory far exceeds that of most d20 setting writers. I desire to plunder the fertile brains of these people. Brains...
On the continent of Faerun, in the 3.x WotC campaign world of the Forgotten Realms, there exists an inland sea by the name Inner Sea or Sea of Fallen Stars. Over this sea, many powerful trading nations exchange goods on a scale far larger than our historical Earth knew until recently, with technology mostly confined to GURPS TL3/TL4 and some magical augmentation.
Nations of the western half of it include Chondath, with a population just shy of 2 million; Cormyr, maybe 1.4 M; Sembia 2.5 M; Turmish 1.7 M and various free cities and loose confederations that make up another million or two. Say about 10-15 million depending on how far inland we count cities and nations. The Moonsea, a lake that drains into the sea by a navigable river, adds perhaps another two million and we can reach north to touch perhaps several more millions with a bit of overland trade.
Then there are several powerful nations on the eastern shores, collectively making up more than 20 million people and increasingly starting to trade with their western neighbours. Those nations rely on slavery (banned in most of the western polities) to some greater or lesser extent. From those nations, overland trade can relatively easily reach another 20 million, some of them fabulously rich.
The nations of the western Inner Sea do trade overland with the Sword Coast (the other side of the continent) and various realms to the South, but overland trade is significantly less profitable. The 600-1000+ miles to the Sword Coast feature smaller cities than on the shores of the Sea of Fallen Stars and much of the land belongs to monsters more than humans. Still, well-guarded caravans remain profitable, especially given the wealth of some of the nations there (Amn, Calimshan, etc.).
Acriculture is incredibly advanced, due to magical assistance, with nations such as Thay controlling their entire climate with long-lasting spells and working huge fields with specialist mages directing a workforce of slaves and undead labour to bring in a yield for good farmland up to ten times what nonmagical TL3-TL4 farming did in our world. Even nations more hesitant to use such titannic large-scale magic still double their yield with clerical blessings and minor hedge magics.
Economic equality is very low, with nobles and warlords often owning assets worth thousands of times more than a peasant makes in a year. A Sembian merchant noble family might have an income of $10 M per year while the poorest farmhands in the same nation earn perhaps $1000/year and most of that in goods and services they need for survival. Ironically, in feudal (although increasingly strongly centralised) Cormyr, the gap is likely to be less, with the income of the royal family being perhaps 1/10th of what a merchant house makes and with effective 'minimum wages' being closer to $1500/year.
Due to several factors; chief among them the presence of nonhuman races significantly more at home underground and with mining than humans, the past use of powerful magic and abundant natural resources; the price of gold and silver, for example, is much reduced from our world at a similar technological level. Silver is worth $250/lbs. and gold $2500/lbs. Various gemstones, while much more plentiful than in our world, are also important components for magic and the demand for them outstrips supply by a comfortable margin.
Sailings ships are solidly TL4 and have been for a while. Some poorer nations are still relying on their TL3 fleets, but generally, even those have early TL4 ships. Cormyr, Sembia, Chondath and other rich nations have the ability to build sailing ships that span the entire TL4 on Earth and even early TL5 (although the wooden sailing ships of early TL5 were not significantly superior to their predecessors).
Magical transport is usually not practical for bulk goods, since few wizards can transport more than perhaps a 1000 lbs. at once (and most who can Teleport at all manage something closer to a few pounds of extra weight) and a reliable Teleport of that size might cost $50k for one casting (a personal one might be closer to $5k-$15k). The chance for a lost shipment or significantly damaged shipment would be about 1% if shipped to a familiar location, perhaps double or more if shipped to a seldom visited one. Given that this would also land the wizard in hot water, requiring him to refund might be impractical. The chances of some damage to a large shipment might be closer to 5%.
Added to that, no more than perhaps two dozen wizards over the entire Inner Sea region would consider Teleporting goods for payment on a regular basis (the mage generally has to come along and while the chance for fatal or crippling error is small, it adds up to a certainty if you do it long enough). Still, this means that small goods of fabulous value, such as gems, can be brought to whatever place they'll fetch the most relatively easily, as long as one doesn't mind paying through the nose.
Monsters infest the deep waters as well as the wilds. There are also creatures living underwater that are not hostile to humans, such as the merfolk, tritons, aquatic elves and other sea-people of Kingdom of Serôs under the waves. Serôs has a population of under one million people and survives mostly by seaweed farming, fishing, fish-farming and priestly magic. Just three years ago, a crusade of the evil races of the deeps (sahuagin, kraken, malenti, kuo-toa, etc.) forced the surface world and Serôs into temporary cooperation and that has now borne fruit for the last two years in a very profitable trade relationship between Cormyr, Sembia and Serôs.
So far, this trade is a very specialised one and usually carried out with the aid of magic, but the two worlds have much to offer one another. As examples, the price of pearls in Serôs is about 1/10th of the surface value, or less for common specimens, while Serôs mostly cannot make good steel weapons without using powerful magic to do so (Everbright spearheads which cannot rust are some of the most desirable trade goods). Connoisseurs above and blow the waves value new artistic styles and methods, centuries of shipswrecks have left the sea-people with plenty of debris that is essentially useless to them, etc. The magic of the two worlds is also different and mages from both lands pay dearly for scribed examples of new spells.
Gunpowder is an alchemical substance that requires magical knowledge to make, and so has been limited in its application so far. The ability to manufacture it was by edict of most of the major Gods confined to the far East and isolated mavericks until about 14 years ago, when the embargo was broken due to an upheaval in the divine realms. No major army yet fields large musketeer contigents, although that's due to change within the next few years. Seaborne cannons are used only by pirates and Thayvian navy ships so far, but are available for sieges on land to most of the larger nations, if they are interested (Cormyr mostly isn't, given that the capabilities of their War Wizards are more mobile, cheaper to deploy and no less destructive).
Being a self-employed or guild wizard is generally a Filthy Rich job at least. Even relatively minor apprentices rate Wealthy. Despite this, mages seem to pursue other occupations than the pursuit of wealth surprisingly frequently. This may be due to the influence of Mystra, the goddess of magic, and Azuth, the god of mages, who often steer mages towards goal comprehensible only to their divine selves.
The ability to work magic is inborn, but it's unknown how many are born with it, since it requires fairly specialised knowledge to spot the talent. Some sages theorise that maybe as much as half of humanity has the ability to manifest spellcasting, but given that some of those will always lack of schooling, innate intelligence, dedication or interest, it seems very hard to harness all those mages. Organised schooling, strong guilds and societal acceptance of mages does yield a much higher concentration of them, lending some support to the theory that the ability to work magic is much more common than the number of mages.
Clerical spellcasting requires only devotion, not an inborn spark, but is no more common than the wizardly version (few people combine the required level of devotion with the required understanding of the God's vision). Some healing magic is available to everyone in good standing with the church and/or with funds to spare. Most churches are fabulously rich due to donations, with real tangible benefits accruing to the faithful, giving at collection time is usually brisk. Large scale plagues are rarer than on Earth at any time but the present and when they happen, they tend to kill people in the hundreds or a few thousands at most, not tens or hundreds of thousands.
Perhaps 2% of people are able to use spell-based magic of some flavour (with another 1-2% or so with other special abilities, mostly related to Imbuements or supernatural heroism and fortitude) and one or two in every thousand can call themselves a wizard or priest (as opposed to acolyte/apprentice/hedge practisioner/alchemist/etc.).
Sailors are paid from $3k-$12k per year, with the value weighted toward the higher end for experienced and capable sailors. Most every ship requires a marine complement and a ship's wizard, meaning that in addition to a full crew, it's often necessary to pay $100k-$500k per year for their salaries.
With this background out of the way, I've got a couple of thoughts. How many tons of cargo would it be realistic for Sembia (trading hub with 2.5 M people and incomes between $1k-10 M, with the median yearly income close to $6k) to ship yearly? What would be a good $ value for their yearly trade? How many ships do the main trading nations have, compared to their populations? How big are those ships? Does it make more sense to have a few large ones or a larger number of smaller faster ones? What's the proportion of military vessels and civilans vessels? What's the minimum value of a ship's load of cargo to make it profitable to ship? What's the maximum believable value? I realise that this depends on a host of variables, but if someone has time on his hands and interest in the subject, seeing someone else's thoughts on what those variables would do would be interesting. What goods would be shipped by magic? How fast is it likely that the trade with Serôs would grow? How much volume in $ per year in the first year, second year and how much will it be worth in ten years? If we assume that the kingdom of Cormyr has access to high-quality sapphires in staggering numbers, say perhaps tens or even hundreds of thousands of stones, how much would that depress the price of sapphires? If we assume that they have successfully kept the extent of their stockpile secret, attempt to sell through intermediaries everywhere around the Inner Sea and beyond and maintain a slow pace, how much would they have to sell in one year to have a significant effect on the price? Would a thousand stones at $25k-$150k (average $50k) each sold all around Faerun over a period of two years depress the price catastrophically? Any other thoughts on the economics of the world? Anything strike you as weird, unexplained or impossible? Anything else people need to know to comment?
Trade Goods Most Wealth is not in coins. It is measured in livestock, grain, land, rights to collect taxes, or rights to resources (such as a mine or a forest).
Guilds, nobles, and royalty regulate trade. Chartered companies are granted rights to conduct trade along certain routes, to send merchant ships to various ports, or to buy or sell specific goods. Guilds set prices for the goods or Services that they control, and determine who may or may not offer those goods and Services. Merchants commonly exchange trade goods without using currency. The Trade Goods table shows the value of commonly exchanged goods.
Trade Goods Cost Goods 1 cp 1 lb. of wheat 2 cp 1 lb. of flour or one chicken 5 cp 1 lb. of salt 1 sp 1 lb. of iron or 1 sq. yd. of canvas 5 sp 1 lb. of copper or 1 sq. yd. of cotton cloth 1 gp 1 lb. of ginger or one goat 2 gp 1 lb. of cinnamon or pepper, or one sheep 3 gp 1 lb. of cloves or one pig 5 gp 1 lb. of silver or 1 sq. yd. of linen 10 gp 1 sq. yd. of silk or one cow 15 gp 1 lb. of saffron or one ox 50 gp 1 lb. of gold 500 gp 1 lb. of platinum