Like I said I am small time, my trailer is only 7x16ft I have vended at what I believe is our full output capacity meaning we were putting food out the door as fast as we could and we only grossed $2400 for an event from 11am to 4pm. The most I have ever grossed for a 3 day event is $5000.
But to answer your question if I did an event and grossed $8000 and the next years they wanted $500-$600 yes I would pay it but if they wanted 30% no I would not. The only way I could pay the 30% is to jack the price and I am not one to charge $8 for a sammie or $4 for a bottle of water like many have to do at these events.
But there again I do not vend festivals much, only 3 to 4 per year and all are local.