I had this problem also. I showed a few ladies at work a scarf I made a few months ago when I first learned to knit (
scarf). They loved it, wanted me to make more, and one wanted to buy the one I brought in right then and there. She asked how much, I said I had no idea, she offered $20, I said "Sold!".
Yeah, because of the fact that I am by no means a fast knitter, $20 by no means covers "labor" cost (even at minimum wage!

), but since I sit around watching movies while I knit, getting paid
something for an activity I like to do for free anyway, is better than trying to get "fairly" compensated and selling nothing.
That said, charge what the market allows. If a detailed scarf can only sell for $40, but a much simpler one can sell for $20. Focus on the simpler one. Of course, if you can get $100 for a scarf, more power to you!