Unless I'm mistaken, 1 yearly papal influence vs 5% cost increase is a net gain unless your yearly papal influence is already at least 20 (with the current maximum being about 11). Of course, the additional +1 is stronger if your current yearly influence is low. You'll notice a larger difference as an Irish minor without cardinals (and not one of those with +2 in their national ideas), than as Spain with 7 cardinals, religious ideas and defender of the faith.
Personally, I usually take it if I have a free slot.