I recall a blogger I follow writing a book (10? 15? years ago) and asking people who were buying it to use his Amazon referral link. His affiliate payment on each sale was higher than his royalties.
I'm not sure if it's completely standard, but my royalties have always been 10% of wholesale (which usually works out to about 5% of retail). Amazon will pay out 5%-10% referral fees depending on the item and how good an affiliate you are. Also, you can often earn royalties for other items bought in the same session, so Amazon could well be a lot more lucrative.
