@microwater: Many of them are in college so they get to work at your local shop and make a penny on the side while parent's pay form most of the stuff.
However, MOST of them make a pretty penny on the side by making the photo-shoot. Gives comic-cons fans a nice girl with a nice smile and a nice body, and they will have her in a jumpsuit, taking pictures of her at every turn. Last year, I was at a Comic-con which had two spokes-girls in body-latex suit for a vendor, and there was a crowd big enough that literally blocked off isles.