Well you could accept a price, or you could accept a *contract.

*I always ask to be able to read the book, before i design a cover for it.   The entire book is not nessesary but enough to know if its actually good or not.   In general, what you should do, is ask to be included in comission of sales (just like the author is) that for every book purchased, you receive a dividend.

This is actually the normal professional practice.  An author gets paid by how many sell, and so should you.  Think of the bargain he'd be getting if you accepted 300 bucks, yet he sold 3 million copies getting a commission of 50 cents per paperback sold.

And its your art thats selling his book.