Expensive how? If its just a once of cost then in the scope of things its not expensive. If it is a reoccurring cost then the priest need to have a large income and carry around gold to pay them. So they are carrying around gold just so they can .... pay to reduce the risk to carry around gold. Perhaps the system isn't random? Perhaps it targets those that carry around large amounts of gold?
I would presume it does exactly that.
Could also have some random factor based on control/loyalty of a region; that would make a lot of sense.
