We rent on chegg or buy on
Amazon. We crunch the numbers over and over (both my bf and I are in school right now) to make sure it's the right price.
Another thing I do, and this only works as much as you want it to, but I do swagbucks to earn Amazon giftcards. If it makes sense to buy our book on Amazon, I do it with even less out of pocket. Then the next semester they might buy it back (usually) and the money is in Amazon credits. We're going to be in school for the next million years, so the money is just rolling over and over ... sigh.
For example, I just bought a book for semester B for summer (like three weeks ago, I think) and it was $12 out of pocket (including a few other things we needed off amazon, I think the book was $15) and amazon is quoting buying it back for $12 ... Technically out nothing for the book.