It really depends on the specific petites; some are pretty rare and can be expensive. That said, petites aren't quite as popular as regular ponies so the prices can be erratic (very high if there's a bidding war, on the low side if buyers are scarce at the moment). The later-year petites tend to be more expensive: I'm forgetting specific set names, but the ones with glitter inside, the ones that glow, and the neon ones are harder to find and more expensive. Oh, and the G2 toy fair petites are super rare and expensive. Hope that helps somewhat!
I know we have some petite experts who can give you a better idea.