On the same subject is there a way to alter missionary costs? Basically I want to see the effect of dropping the price and lowering the success chance.
You can change the chance of success in "religions.txt" for each religion:
#missionary_placement_chance = X.XX Base chance for conversion (default is 0.10 if not specified).
#missionary_placement_penalty = yes/no If yes, provinces of this religion give a penalty for missionary placement chance (default is yes if not specified).
and if you make it less likely to succeed you automatically will need more tries. That means even with the same price for one try that the cost for a successful conversion will be higher as you need more tries. So no need to make each try more expensive too.