I recently bought an ODE text (Fundamentals of Diff Eqtns - Nagle 8ed) a few days ago. The book came and it was the wrong edition (6ed). I contacted the seller and now he is willing to return a portion of my money He mentioned at the end that "I think refund a portion of your money would be easiest. I don't need the textbook for anything so at least you can get some use out of it. The text cost me 18$ to post. How much is it worth to you?" I paid $86 (including mailing fee) for the book, how much should I bargain to get the most out it for my book?