I'm going to be blunt with all'y'all. You are most likely under-pricing your work. That's okay, but please consider the following:
Industry Standard Price List for Book Illustration*Black & WhiteSpread: $500-$1500Full Page: $500-$9500Quarter Page: $100-$350Color
Spread: $700-$2000Full Page: $500-$1500Quarter Page: $200-$400*From The Handbook of Pricing and Ethical Guidelines by the Graphic Artists Guild, ed. 13--basically the bible for all visual artists looking to sell and price their work. Check your local library for a copy.
Not all artists are comfortable charging this much for their work, or maybe feel that their work isn't goo