The primary reason textbooks are expensive is not "marginally useful CD-ROMs and other supplements," but simply that students will pay these prices. Why will students pay them? Because they are critical to succeeding in so many classes. Why are they critical to succeeding in so many classes? Because professors build their classes around textbooks. Why do professors build their classes around textbooks? Because publishers make life easy for them by writing canned exams, study guides, web supplements to (even replacements of) traditional lectures, etc. Professors, in turn, have little incentive to care what their students pay, and so publishers can get away with all of this. Publishers also use the new-edition tactic to gut the used-textbook market, which would otherwise eat a lot into sales.
I teach 5 classes on a regular basis. In one I use no text at all, in two others I use standard textbooks but make them optional, and in the other two I have mandatory books, but books which are not traditional textbooks but mass-market books by Thomas Sowell that are a lot cheaper but do not have "problem sets," web supplements, etc. But that requires that I create and grade my own tests, not lean on the publishers' add-ons as busywork for students, and otherwise actually aggressively teach my classes. It is old-fashioned, but students appreciate it. There is some justification for the traditional textbook model in some science and language classes, where repetitive problem-solving and/or drills are important, but the fault otherwise lies with professors who don't want to do the work of teaching.
Our son is in grad school, and it seems that since he’s been in classes at that level, fewer profs require a “standard” textbook, and more either use various books with readings on the subject; post the material or links to articles they use on their “blackboard” online, or use no textbook at all.