Dear Sue
I have been doing craft shows for over 20 years and have actually made a profit and enjoyed a number of vacations on the earnings. My rule of thumb is basically take the materials and times it by either 2 or 3 depending on the amount of time involved to produce the item. I sale mainly stuffed animals, small quilts, and household items such as potholders/coasters/microwave bags. My big quilts do sale but only on the rare occasion and usually to someone the knows and appreciates the amount of work involved in producing a quilt that is full size or bigger.
In Arizona, you are required by law to be a licensed business where you collect and submit taxes to the proper jurisdiction. Be sure you are up on the laws because here the state officials do show up to the various craft shows, collect your license fee and give you the proper paperwork to account for your earnings that day. Then you are on their list FOREVER! Though you may only earn a few 100 dollars at any given show, they expect their fair share.
But since I'm licensed I also receive tax exemption on my supplies & fabrics and in some places I get a vendor or commercial discount on top of that. Give and take. Just be up on your local licensing and tax requirements. And have fun - I've met more wonderful people in all the shows I've done and will continue to enjoy doing them as long as God allows me the strength and talents to do so.