The last thing isn't true anymore, I believe. It never works like that in my games now. And about the amount of money, see, if Muscovy has only 1 province that's Muslim at game start, and you support those rebels that are only in that one little tiny province (Kasimov), you will pay little money for that, because the rebels' base is small. But if Muscovy, let's say, conquers the whole of Kazan, and there are many unsatisfied provinces, many many, then supporting them will cost more, because there will more more provinces/people rebelling, the uprising will be bigger in their numbers. That's the logic, I guess.