Request for help: Royalty Rate Assistance

This blogger has been asked by an author of a book who wants to sell the copyright in his book in exchange for an ongoing royalty on the income (if any) generated by the publication/sale/exploitation of the book/copyright. He has enquired whether there is a database for such royalty rates or any guidelines to determine a market related royalty rate as compensation for such an assignment of copyright. Is there a reader who is able to point this person in the right direction?

Comments appreciated.