I think it's a pretty good head, and I really like the shading. What I don't like is that the curves aren't so much curves as they are series of lines. I don't know if this is what you were going for, I personally think it would look better as smooth curves than sort of blocky-ish lines. Also, I think there's a seam at the back near the top, might want to fix that. I'm not going to pretend to have any idea how much it's worth.