This site, which I wish had been available when I was in school, helps students reduce their textbook expense. They sell Clone Reports - a $15 doc that summarizes the differences between the new and old editions of a textbook. The reports identify which textbook have been updated with little value compared to the potentially cheaper previous edition.
You might need to be an active student to have noticed, but I can recall from my own days that virtually every site which sold textbooks usually listed the previous editions. For every $150 textbook there is usually a previous edition available, and you can almost always find it for under $10. Sometimes the price gets even cheaper, especially for the Bio, Chem, and other 101 classes that millions of college freshman take every year.
And now with Clone Reports students can buy that older cheaper edition and potentially save quite a few dollars. The name of the reports is something of a joke; it stands for Content Lacking Originality in New Editions. Each report contains details on the changes made to the text, images, and self-test or problem sections, and it also includes a Rayscore which gives a specific percentage to how much content is the same between any 2 editions.
Raygun currently has a number of reports ready to buy, and you can download them in PDF format. If they don't have the title you need you can request it.