Depends on the artist. Some have a set fee. I talked to Dale Thompson via e-mail, and he wanted a small fortune. Scott paints houses for a living, and he simply charged me his day rate. But, some people will work for a cut of the publishing/writer's credit. It just depends on who they are and what you work out.
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"I have no desire to speak w/other tongues, I already do enough damage w/the one I have!!" - Candlemass