I'm kind of in the same dilemma right now. I was given some advice from someone recently on what to do. First figure out the overhead amount (what you paid for the fiber), then figure out a amount/hour that you think is reasonable to charge. Finally any extras like if it's 2 or 3-ply yarn, was it hand dyed, etc.
I have a spreadsheet I started 2 days ago to figure this all out, and basically I'll have a column for everything, (one for overhead, singles, 2-ply, 3-ply, extra ounces above 4, hand dyed) plug in my numbers and it spits out a price. It also acts as double duty for me to figure out my inventory as well.
Hope this helps.