The main reason textbooks have gotten so expensive is used books. Textbook publishers can no longer rely on a textbook selling consistently over the life of its edition; once used books start entering the market, fewer new copies get bought, which means the price of new books has to go up. Simple economics really.
Another problem, as others have pointed out, is that students are importing titles from overseas markets where the price of textbooks is more strictly regulated. Result: the publisher is again being undercut by its own product, ergo the domestic new book price goes up to make up the shortfall.
Anyone who thinks publishing textbooks is a license to print money needs to pull his head out of the sand. Writing textbooks is a very good racket; a successful book can make an author a millionaire several times over. But textbooks cost an insane amount of money for a publisher to develop; in addition to the book itself, there are numerous supplements and teaching aids that students never see but that profs rely on to teach their classes. (Plus all the student study guides, websites, CD-ROMs etc that most students ignore despite the fact that they’ve paid for them.)
And your bookstores aren’t laughing all the way to the bank either; sure there’s a markup on textbooks, but no more than for any other product you buy in a store, and it’s often less as a result of pressure from both students and faculty to keep prices low. You want to get rich? Don’t open a college bookstore.
And whoever made that remark about books being revised solely to milk students, get a life. How would you like to sit down in your biology or economics class and find that your prof assigned you a book that’s five years or more out of date? How much success do you think publishers would have getting profs to adopt books that were five years or more out of date?