It's sort of chivalrous for the guy to pay for his own protection, but in a practical sense, I think that the money spent on protection should be about equal -- this includes birth control or whatever else. If the guy is paying a lot for condoms, and the woman isn't paying for anything, it's sort of unfair -- but really, why are we thinking of sex in terms of balancing numbers? Just have whoever is willing to go to the store pick some up on the way back. Ideally, it shouldn't be that big of a deal and should equalize out over time.
Alternatively, as someone said, both people buy them and keep them at respective living places (or, just have extras! You're going to use them someday anyway.)