Funds rating varies! On the record, Mentats take a Vow of Poverty. Wealthy benefactors include Emperor Gregor Vortapani and the Mentat Force Order
Also note that an adventuring Mentat typically is in service to a specific small organization or individual, most often a Vor House or high-ranking leader of a noble household. This employer's priorities will override personal concerns in some cases.
Kit Kat and another graduate of the Mentat Academy, Almon d’Joy, were once fairly friendly rivals in school, but Kit Kat was gifted to House Vorpadaran and particularly Danar Vorpadaran, and Almon was gifted to, well, other persons for whom his skills were more closely aligned. You see, Kit Kat, often concluded answers to problems faster by immediately discounting unethical courses of action, thus reducing the number of possible solutions to be computed. Almon took the other route, often arriving at more creative solutions, but less ethical ones.
However, near the end of 12726, by Tapani Year recoging, the events of Pitch Dark transpired. This brought him to the attention of Tapani Imperium's Emperor Gregor! The Emperor confiscated Kit Kat through Imperial Fiat to work on larger projects that affected the Imperium as a whole.