Well... it's not a "commissioned" image so it would have existed with or without the offer. Take it, it's free money :D Don't worry about selling it cheap, if it's your first sale (and it sounds like it is) then you want to get your stuff visible for your portfolio. Overpricing it might lose you a sale, and saying to prospective clients "I nearly got a picture on a book cover" isn't nearly as impressive as showing them an image of a book using your work.