Publishers only make money from new books, so they have moved to releasing new editions in a two-to-four-year cycles,which keeps the cost of book rentals and used books higher due to their shorter life cycle.
Professors are often unaware of the cost when assigning the latest versions of textbooks. Most new editions contain only small changes, like newer photos and somewhat updated cultural references, but enough to change the page number references.
Publishers attach etext and supplemental materials to textbooks that might look like a good bargain, but raise the price. A study by the California State Auditor found that the majority of students have had to purchase "bundled" materials they never used for 2 to 5 courses.
Michele Van Hoeck