Attached Link: http://www.gag.org/

Congratulations on your covers! This is hard work to get. Prices vary wildly depending on region, industry, your reputation & experience, licensing, and so forth. Listen to jshoen, he's on the money. FYI, please check out the Graphic Artists Guild Handbook of Pricing and Ethical Guidelines. This is the BIBLE of the working graphic artists. They even have sample contracts. Be polite to your publishers, but try to restrict their usage of your work, or at least define it properly--things like first North American or European rights, define the length of time they have usage (years), and where else they may apply the images. If they are using your images in identity work (logos) or advertising, you may want to ask them to let YOU be the one to change or re-layout the images to suit the application. And you can charge more for identity and advertising/marketing than for just covers. But, sometimes, with very small presses, you can ask for some percentage of royalties if they just don't have money up front, and they may agree. Of course, you have to chase your money after that, which is a bore, but it can ingratiate you with them. Best wishes!