If you can't make money selling snacks and drinks to 100 fat, captive gamers then honestly you have no place being in business. Add in board game / card game / dice dame / accessory sales from bored, between-round browsers (which means you actually have to have something people want to buy) and sales from normal people who come in (and again, if your play space isn't big enough to accomodate the event AND shoppers, what the hell are you doing hosting it to begin with?!?) and you should make more than enough.
Seems to me the clearest way to alienate this community is trying to make $5-$10 a head on the entrance fee and not offering decent prize support.