In the world of the small and cooperative press, there can be exceptions to the standard book contract concerning payment.
Standard contracts offer a fixed royalty rate and a fixed royalty, or percentage.
The rate is based on the retail price of the book when published with a major publishing house. Most small presses offer an exception, basing the rate on net receipts of the sale of books to wholesalers or distributors. The royalty is the percentage of money the author or copyright holder receives per book sale. Any contract must reflect the recovery of the funds the author contributes. Beware of the tricky alternatives.