This is a really good thread!
First off I don't know if it'll help anyone but I have an etsy fee calculator at my website... those fees really add up if you're not watching!
http://spacefem.com/etsy/feecalc.phpI try to get a 30% profit margin on my supplies that I sell. That's enough that when an occasional one gets lost in the mail, I don't panic, I can send another one and still be ahead for the month. But I've also noticed that my prices aren't too bad with that guideline.
On handmade items, that's trickier, because what's your time worth? I tend to price based on my competition.
One tip I've learned along the way... set your prices high, but have a clearance section. If your clearance items are flying off the shelves, maybe you need to lower your prices, because people love your items at lower prices. If people aren't buying even your clearance stuff, well then it's nice the prices keeping them away, you can be confident keeping them where they are.
And my last tip... keep your accounts separate! Buy your craft supplies with paypal, deposit your etsy direct money into a special business account. That way at the end of the month, you know that you're ahead because all your revenue minus what you spent is right there as profit. Helps make sure the little expenses like shipping labels, thread, whatever aren't adding up without you noticing.