I haven't had any successful experience with providing products to affiliates, but I'll share my thoughts... I have been a writer (books like QuickBooks for Dummies) and also a book publisher. And those experiences in the book business, make me say that the percentage should be something less than fifty percent.
I.e., in a regular bookstore where the retailer actually has to buy the product and then inventory and take care of the product, the retailer's share is fifty percent (or actually sometimes a little less).
A "retailer" who doesn't pay up front, doesn't provide sales people, doesn't process the refunds, etc., would logically seem to deserve much less.
I thought about the fifty percent a retailer hopes to get... about the fact that the percentage could not be zero. I guessed that 25% might be reasonable in my case...