For me, this is more of a numbers game than anything else. If I know my profit margin against gross, then I can determine how much I can give. Most food industry numbers suggest a profit margin of single to low double digit percentages. If I am making a profit of 8% of gross, than I am not going to give up more than 5% of gross and probably no more than 2%. Captive audience means nothing to me, nor does being the only BBQ vendor. If I am the only food or beverage vendor, than we can talk higher percentages of gross, as my profit will be higher.
When I shot professional video for horse events, we were regularly promised exclusive site rights. Yet folks brought their own equipment, or worse, their own shooters. Often we would be working for a loss, and the event organizers never ever sided with us. Percentage of gross always have to come down to how much profit you can make.
As for your question, I think I am willing to give up 25% of profits, which means, if I make 8% of gross as profit, then I pay 2% of gross for sales. Personally, I don't like being taxed by event organizers.
Whip It Off, Chambers!
"perhaps...but then again...maybe not..."