Illustrating Book About Chinese Zodiacs

Dear Rich: I'm an artist and my friend is a writer. She wants me to illustrate her book on Chinese zodiac astrology. My art would be heavily displayed in the book. How do we work out my getting paid for my art? Do I charge her a flat fee or take a cut of the earnings? Consider the following questions before proposing anything to the writer.
Is the artwork supplementary or essential? If the artwork is an essential part of the purchasing decision -- as in a home decorating or children's picture book -- that would be more likely to justify a cut of the earnings. If the illustrations simply supplement the writing (as in most nonfiction books), flat fees are more common.
How much time will it take and what is your time worth? Estimate the number of hours per drawing. If you were billing an anonymous client for the same work, what would you charge? This would be your starting point in determining what you want to earn back (though of course there may a steep discount for your friend). The Graphic Artists Guild Handbook provides assistance with pricing.
How likely is it that the book will be distributed by a commercial publisher? If this work will be the subject of a book deal, the typical advance (assuming there is one) for a nonfiction book would be between $3,000 and $10,000 (though others indicate these advances are higher; we don't think that's an accurate reflection of today's marketplace). Royalties would be 5 to 10% of the book's income but would only be paid after the advance has been earned back. Nonfiction books usually have a sales life of 12 to 14 months. If a book deal is in the works, and you feel you are entitled to a cut of the earnings, you should seek to become a party to the publishing contract. Otherwise, you must rely on the writer to receive the income and then forward your payment.
What does your friend want to do? Because you're dealing with a friend -- not an anonymous client -- we'd like to see you work out an arrangement that doesn't jeopardize your personal relationship. You could propose a two-tiered system -- for example a reduced flat fee but an additional payment if a book deal is signed.
What about the rights? Are you transferring copyright ownership? Do you want to retain certain rights so that you can continue to use, sell or display the artwork. Determining rights may affect the price. For example, if you would like the right to sell limited edition prints, or to advertise those prints in the book, you might want to adjust your fees accordingly.