The only one I've applied for that had an app fee was $25 which was honestly fine for me to pay. It applied to my booth fee if accepted, and as long as I sign up for the following year at the end of the event, I don't have to pay the fee again. I think they use them to help weed out vendors that wouldnt be a good fit for the event. If it was a nonrefundable fee that didn't apply to booth rent if accepted though, that's slimy.