If the cash flow problem is of a relatively temporary nature, then since obviously everyone you go to school with whose a year ahead or more will have the book (unless they just changed it) then your solution is probably to just ask one of them if you can borrow it for a few weeks, or just long enough to run copies off of the first couple chapters. If it's a situation where the things get used so regularly even after the class then obviously you'll never find a used copy or someone to loan it to yu, but if not, borrow it and buy it when you have the funds. Most physics books, doesn't really matter after the class which text on the individual subject you reference from, so nobody minded loaning a book to someone else because we had other texts that had all the equations and of course, due to the environment, if we just suddenly needed that specific book and it was on loan, we could just grab a copy from the shelf of a fellow grad or prof's office. If your situation is different, well, best adivce I can give you would be to just try all the major bookvendors or even the publisher's site, some sell direct IIRC since grad-level texts always have such short print runs. I really can't imagine a case where any grad-level student couldn't get their hands on a copy of any text they'd need to have by simply borrowing it, even if it's a brand new book than one of the profs who reviewed it for approval has a copy.

Hope that helps, but short of borrowing, I doubt you'll find better than 20% below campus bookstore, except used, and even that's probably optimistic. I'd also highly doubt a grad-level text would appear on ebay except as a hard to find accident, people just don't usually get rid off them and when they do it's usually to the campus bookstore.