It sure is. There are many factors, such as considering the clientele who will show up at these craft fairs. I looked at some acrylic ponchos at The Paper Store recently, and they were charging between $30 and $50 for machine-made ones. That seems a little low for me, given that I spent $18 on the yarn (parts of six skeins).
I'm not sure what to charge. I go by that formula to triple the cost of supplies, and then I add a little extra for my time depending on the difficulty/fanciness of the pattern. I think I'd like to charge $70 or $75. If someone offers me $65 or so, I'll probably take it, because I could really use the extra money. But I won't go under $60, given what I spent for the yarn.