I really think (as noted in this thread), you have to decide based on cost, attendee numbers, number days of the event and the cost of your items, plus any travel time & costs. Mine run in that $5 - $10 range mostly so it takes a lot of sales to recoup booth fees. For those selling higher ticket items, the sales $ add up quite a bit faster.
I try to use the mindset of ...right now it's all about marketing so even though I may not make a lot of sales, I'm trying to get my name out there. I'm doing a show this year I did last year so hopefully I get some repeat customers but last year it was super hot and people weren't buying much of anything. I usually note whether or not they are carrying packages when they come in or walk by. The most I've paid is $100 for a one-day show that brings a lot of people to town, but a lot of that number is for food, music and the other activities.