Well, PSN doesn't cost Sony anything because it's making them profit.. That would be like asking how much a PS2 costs Sony.... But, if you're looking for an answer before gains, I'm not really sure. I heard somewhere on this site they get 16 cents of each gigabyte sold.
People are paying for PSN (or at least I am). I've bought several games, and a couple movies from PSN. I'm sure that contributes to why PSN is free. Because people are paying for it. But, only for what they want.