While I very much appreciate the online "textbook companion sites" that offer things like quizzes and flashcards, the one time I purchased an "access code" for a textbook site, I was decidedly underwhelmed. This was especially true because my access only lasted for 6 months. I just wasn't worth the price, IMHO.
Google, "textbook costs" and you will find dozens of articles of the same theme - "Why do college textbooks cost so much?" It's a problem. Everyone knows it's a problem. Even the GAO got involved with an investigation. Unfortunately, it seems unlikely that potential solutions, such as Flat World Knowledge, will ever go main-stream. I read not too long ago a comment by a college professor explaining the amount of work that goes into writing a college textbook and why they should cost as much as they do. (Interestingly, she didn't seem to grasp the question posed to her; that is, has the amount of work somehow increased so much from the 1970s and 1980s that it would explain the jump in price that is more than doubled the rate of inflation?) My thought, though, is quite frankly, "Just how many introductory biology textbooks does the world really need?" I own 5 introductory geology textbooks and there are many more out there on the market. The thing is, they really aren't that much different when it comes to content. Yes, they have different photos and may introduce different topics in different orders, but for the most part, they all teach the same thing. Perhaps part of the "work involved" on the side of the authors would be to determine whether or not there is a genuine need for the book they intend to write?
One of the great things about using testing to earn college credits is that you do not have a "required textbook." Not only do you save a bundle on tuition costs, you can also save a lot of money with textbooks as well. You have the option of not buying a textbook at all, or buying a used previous edition, often for less than $10 including shipping. When you supplement your studies with resources found online and at your local library, you can gain a thorough knowledge of your chosen subject matter, without breaking the bank.