I check etsy and similar websites to see how much similar items are selling for. If you are selling at craft fairs, walk around and see what your competition is selling similar items for. Stay around them, don't undercut for this helps nobody. You know how much materials cost and a rough amount of time that it takes you to make something. You need to decide if its worth your time to sell an item for the prices your individual market can afford.
I saw for fun, if I can buy my lumber, saw blades, and maybe a new tool off of my sales then great. I'm not going to get rich from using a scroll saw, but I'm not going to let the hobby bankrupt me neither.
My best shows are jurried street fairs. (Just had one last weekend where we made $4000 in 3 days.)
My worst shows are church craft shows. (Barley make $200 a show)