Angela Fehr recently shared a couple of good posts on pricing your art. She gives 7 ways to price your art: by size, by time, by cost, by comparison, by circumstance, by experience, by instinct.
Read the first post: How to Price Your Art, Part 1
And the second post: How to Price Your Art, Part 2
I think she's spot on. We can make up all of the formulas we want, but we have to use instinct to arrive at those formulas. Read her posts and let me know what you think.