You should keep track of the time put into it and decide how much you'd like to earn for your time. Then add overhead, which includes the fabric, batting, thread and a percent of the fee you may be paying for the booth. If you're worried that it might be a little high, put up a sign that you'll reduce you prices by a certain percentage during the last hour of the fair, or just offer that verbally to any customers who might say something like, "I wish I could afford it." If you were to under price things, you would not only be selling yourself short, but you'd be making it harder for other quilters everywhere to get a fair price for their work. Another way to get some idea would be to talk to others who will be selling at the fair, and see what they consider the hourly amount for the time they invest in their crafts.