I have never had to pay a percentage to the event. I would think that anything over 10% and I wouldn't even consider vending there.
All of my events have charged a flat booth space rate between $80 -$800 for a standard 12x16 booth space. Only events that I turned down were ones that had a high booth space rent (over $400) with not much chance of making the profit I wanted. This could easily be figured by taking the number of expected people at the event and dividing by half. Take that number and divide by the number of food booths selling meals ( don't include popcorn, ice cream, etc.. booths). And that will give you a rough estimate of how many people will eat at your booth, ie: 5000 people / 1/2= 2500 / 6 (say there are 6 other food booths) = 416 possible customers. If your average meal sold is $7 that would mean you will take in about $2912.00 Even though this is a rough estimate this formula has been incredible acurate for me on any events that are new to me. Subtract out your costs and fees and see if that leaves you enough profit to make it worth your while.
On a side note, don't forget to figure in sales taxes. Some of the events that I do are considered a tourist event and I have to pay the state an extra 1% tourisim tax, so taxes run as high as 8% for me.
Some people are like Slinkys, they are good for nothing but can still bring a smile to your face when you push them down the stairs.
Down to one lonely ol' Bandera.