Given the prosperity impact, raising taxes can actually be counter productive when including the extra cost for each action in the province - particularly in smaller provinces, or ones where you have low law. You wind up spending 2/4/6/etc extra RP (or 1/2/3/etc GB) on every action just to keep your chances the same.
Hate myself for saying it though I do, convincing your local temple to bless the province is probably more likely to bring in the cash, although you might need to share some of the gain (the law holder gets more income from all types of holding, the temple probably only gets it from one so tends to lose out). Otherwise encourage your unlanded domains to fill up the land - offer to grant law advantages to rule / create actions, etc - for a province ruler an empty holding of any kind (barring sources) is an unproductive one - and while 'the main' domain in the area might not see the slot as a priority you get the same tax from joe shmoe 1-lousy-holding-only regent you still get the same tax from the slot, joe schmoe is also less likely to throw their weight around...
Incidentally, highest rate = prosperity hit encourages blanket rates, a weighted average by domain size would give a fairer result if more cumbersome - the P&H should be able to build in a formula reasonably easily, although thinking on it the 'favoritism' aspect might be negative in itself - and the gratitude never goes as far as the bitterness.
