To make it profitable you would need a lot of donations, cheap hall rental and food, and speakers and things all paid for by vendors. To be quite honest, our swaps don't make money or a lot of money and we don't have to because we are a non profit. But if you get a lot of donations from people coming and you have a good attendance then yes you could technically make some money, not sure how much, but you may be able to pull in a couple hundred bucks.
In my opinion, the vendors make more money than the club does, as well as the hobbyists who sell things at the swaps.
Maybe it is just the way our swaps are laid out and prepared, but if you are charging everyone at the door, and can find someone to sponsor all the expenses, then yes you could do very well.
Give me some more information on exactly what your plan was and I may be able to better understand what you mean and tell you if its feasible. If you include a frag swap in some sort of existing flea market then it probably won't go so well, but if it is promoted at LFS all over Pittsburgh and the LFS donate stuff for a raffle or offer a discount, your attendance will go up.
thanks,
Rick