While helping a student find a web site for used textbooks I discovered a site where students can rent textbooks for a semester. http://www.chegg.com/ According to the site, students can save 65-85% off the list price. It's still not cheap, but can save students a lot of money for books they'll only need for one course. They have a chat service on the site as well.
Here's an example from their FAQ:
With Chegg there are no monthly subscription fees or hidden charges. Each textbook has a specific rental fee, which is dependent on the ISBN. Shipping fees are dependent on the shipment method chosen. The average rental price on Chegg is 60% lower than the list price!
Biology (7th Edition) is listed at $158.67. Rent it on Chegg for $71.40 - 56% savings!
Calculus (5th Edition) is listed at $180.95. Rent it on Chegg for $62.09 - 66% savings!