Well, that IS ridiculous. Even highly paid agents don't usually ask for that much (ten to fifteen was more common last time I checked). The only way that could conceivably make sense was if it was "profits" rather than "sales". Otherwise, as you pointed out, most vendors would actually lose money.
Well, to be fair, I don't know the profit margins for vendors at anime conventions but I have assisted people who were doing things like flea markets and I must assume it's not too dissimilar (or else everyone would be at the anime conventions selling stuff at ridiculous margins).
Let's see, using Google as a brain extension, I found this:
http://www.retailers.com/eduandevents/ask/askprofitmargin.html