The problem is the lack of an open market. Amazon has provided more transparancy through reviews, but there still is no freedom of choice for the consumers when a textbook is assigned. There is virtually no competition on price possible.
Moreover, you can't just choose another textbook, as the topics that are covered may differ slightly. Even in math, when you use another Calculus book, there is a reasonable chance you miss out on some stuff, that just happens to be on the exam.
I'm not sure Seth's solution will work and I would rather propose that teachers should be clearer on what kind of problems the student should be able to answer/solve, while keeping in mind that the student should be able to use any in a range of at least six books to study the material.
The presence of "freedom of choice" for the consumer depends on how you look at the industry. From a student's perspective, there is no real choice - you use the textbook you're assigned. Some professors will offer a few alternatives, but unless they're very diligent about always giving references to each book (and I had a few professors who did this well), the students who use the number-one book - the one the professor uses when he's lecturing and doing examples - will generally have a better idea of the material and the standard they'll be evaluated on.
From the professor's point of view (considering them as the consumer), however, there's quite a free market. They can get free evaluation copies of any textbook they want, and most schools give individual professors the choice of what book they use for a given class. So the problem then becomes getting professors to switch textbooks, which is tough. Most professors - especially in things like science - will teach the same courses year-on, and will become very comfortable with the material in a given book. They'll know what to skip, what to supplement, and what to offer additional comments on.
Moreover, you can't just choose another textbook, as the topics that are covered may differ slightly. Even in math, when you use another Calculus book, there is a reasonable chance you miss out on some stuff, that just happens to be on the exam.
I'm not sure Seth's solution will work and I would rather propose that teachers should be clearer on what kind of problems the student should be able to answer/solve, while keeping in mind that the student should be able to use any in a range of at least six books to study the material.