This has nothing to do with income.
If you're ferrying around in excess of 8,000-10,000 gold, you can afford to pay for some protection.
And if you really are the poor priest, with the lowly income, who can't afford to pay, then you don't need to pay, and can ignore it.
It should be pretty regularly available if you are carrying significant sums of gold.
It should also be calibrated in size to give it a neutral expected return at some "big but not insane" threshold; say 2,000 or 3,000– so if you're carrying less than that, it's "not worth it" unless you're relatively risk-averse, and if you're carrying more than that, it is worth it.
I think calibrating it to 8k or 10k would be excessively expensive, but calibrating it to 2k or 3k might make sense.
