I found this graph at the College Insurrection blog, a project of law professor William Jacobson.

When I first started university teaching, I never bothered to wander through the bookstore and see what the books that I assigned actually cost. Once I did, I was suitably shocked.

After I had taught a little more, I began to realize that, for instance, the differences between the “fourth edition” and the “fifth edition” were not exactly substantive.  They mainly existed to damage the used-textbook business, I think.