The problem is that the average textbook sells a small run of copies, and doesn't benefit from economies of scale. Unfortunately, it seems that the professors and the publishers both like the current system, and the students have very little influence on the process.

There are some solutions, but the incentives don't seem to be in place to encourage them. The best seems to be reducing the overall number of textbooks, allowing each one to have a larger run and thus to better benefit from economies of scale (and be cheaper to buy at the same profit margin for the producers). But to achieve that we would have to stop letting professors choose their own books, and professors are not known to appreciate being herded.