I think it will be different from event to event. Some wont take anything and only charge for booth space. Some will charge for booth space and take a percentage.
For example. My local fair is $400 for booth space for 5 days. You keep 100%. Then there is a local little Rib Fest that I will be doing and their booth space is $150 for the day and they take 15% of the money made from tickets that buy your food.