If you pay 10% of the price of the book, I'll consider it.
Do you mean discounted price?
If a book is being sold for 100USD, I would try to get it for him if he is willing to pay me 10USD.
If a book is being sold for 200USD, I would try to get it for him if he is willing to pay me 20USD.
Like I said, I'll consider the job if I get paid 10% of the price of the book.