While this isn't official yet, it looks like Street Fighter 4's alternative costumes will cost you some coin.

A scan from a PS3 magazine noted this and it was pointed out here on the Capcom Unity forums.

Some people are up in arms about the potential cost of getting these costumes, but keep in mind the alternative outfits aren't going to affect your game play one way or another... unless you plan on distracting the other player with Chun Li in her evening dress.

And since we haven't heard an official announcement, it's possible you'll be able to get these through additional avenues in the future.