Ah, but the question really is Tani, since a prospective heretic would gain functionally the same funds as they would before (since they wouldn't suddenly gain a large bloc, and if one did arise it'd pose the same question to each non-holy city member) is the political consequences of that economic cash-grab worth the financial, political and existential costs such an action would precipitate.
Ergo, a heresy inevitably would result in a) a loss of political support which can be costly economically in addition to tagging a player with other substantial penalties b) the actual material costs of a conflict, hot or cold that emerges, which would negate any financial advantages and incur a substantial expense in and of themselves and c) is a payer willing to risk his countries continued existence in the process of apostatising at the hands of a righteous crusade in which those who remain faithful split the spoils to their mutual financial benefit.
These considerations being made, the question boils down to an assessment of whether association with the master of the Holy City is in the interests of the player. Needless to say, considering my impeccable record of staying loyal and supportive to my allies, and the precedent in every papal game I've played of running a pax Vaticana to the benefit of all involved economically and politically, and of laying low enemies of Catholicism. I would hope my fellow Catholics understand that their best interests lie with the One True Faith. Afterall, its why Xinjiang converted, and conveniently got resurrected from the dead all those IOT's ago