HLF, in those classes where you use one or two chapters from a book, wait to buy it and find a bunch of people who are willing to split the cost and then just photocopy what you need. When you sell the book back, just have an agreement to split the money earned on the buyback.
My entire time in grad school, I was afforded the luxury of being able to check books out of the university library for an entire semester instead of just a month like everyone else (perk of being a grad student), so I checked out as many of mine and my girlfriend's (now wife) books as possible. Over the course of three semesters, we save over $500.