Chatting to priests should produce a menu of the things they offer in exchange for donating. The menu clearly shows how much gold is required for each donation, and indicates ones that are too expensive given how much gold you currently have. Selecting an option donates that minimum amount.

One reason for doing this is that it’s very difficult for an unspoiled player to learn that you can even buy protection from priests, much less the formula for how much gold should be donated. This would also enable adding in other variables beyond simple experience level, which is currently infeasible because it would make knowing the right amount to donate too complicated.