Yeah, a small cost would be good. Would also keep down the ridiculous amounts of prestige and piety people can get.
Now I vaguely remember what I was talking about - in the VIET thread Bertouch was telling me how CK2+ had large piety costs for waging holy wars, which didn't make sense to him (and me) because, well, yes, it might have balanced things out, but it's a holy war, why would you be losing piety? So actually that's not really related at all. Well, sort of. I would say the holy wars maybe have a prestige loss rather than a piety loss. Losing piety to gain piety kind of doesn't make sense, even if it's a good balance tweak.