As a freelance visual artist how would I get paid for a project involving illustrating a book cover?

As a freelance visual artist how would I get paid for a project involving designing/illustrating a book cover? I had offers to illustrate covers for upcoming authors books. I was wondering would I get paid off of royalties? Or is there a way to get paid a certain set rate if the book doesn't do well or could I get a percentage of the continual earnings or profits of the book?