In response to this post on Blog-Her, textbook sales are a complicated and dirty little business. Without knowing the specifics of the text and class on the receipt, I can pretty much tell you why her book costs so much.
First off, book publishing isn't good business. The margins aren't that great, so the publishers are always looking for ways to squeeze a few more pennies out of consumers. Bookstore margins aren't that great either, but they get to double-dip with book sales. Let's say you buy a book for $100. At the end of the semester, you sell it back to them for $50, and they re-sell the next semester for $75. That's $25 profit without taking into account the mark-up. Publishers, though, don't see profit from this re-sale market, and it drives down the sales of new books.
So, in response, publishers have come up with lots of ways to force you to buy new books. One way is to produce a new edition every couple of years, on the theory that faculty will require you to have the most recent edition (more on this below). Another way that they do it is to create books that are not re-usable, such as lab books (and, from the context of her post, I’ll bet that was a lab book). Since you are required to buy a lab book, but your lecture text might end up on the re-sale market, the publishers charge an outrageous price to make up for what they will lose elsewhere. The bookstore, on the other hand, makes money either way.
Universities don’t always have clean hands here, either. Publishers will often encourage departments to mix-and-match their own editions of work. If you see a textbook entitled “The My University Name Introduction to Basic Whatever,” that probably means you are paying a premium for an edition that is put together by the department. The department gets some kind of kick-back (er, they don’t call it a “kick-back,” they call it something else), and they are encouraged to create a new edition (i.e., slightly revise the content and/or introduction) in order to force students to get new editions of books, thus increasing profit for both the publisher and the department.
Individual professors are on your side, though, right? Wrong. You know all those lectures that your professors spend hours preparing? Well, in certain fields (the sciences particularly), publishers will bundle prepared lessons (complete with scantron tests, PowerPoint slides, etc) for an entire semester. In other words, some lazy professors prepare little of their own lectures, instead relying on pre-packaged lowest-common-denominator stuff from the publisher. I have a friend who likes to point out that if he wanted he could completely automate every aspect of his class, from the clickers to the pre-packaged online tests, so that the only bit of work required would be his lecture, which would be him reading from a prepared script, and the lab, performed by his graduate students (in my friend’s defense, he uses his own lectures / slides / tests, but he is in the minority in his department).
Of course, there are often other compelling reasons to use pre-packaged tripe: Some departments use them as quality control when most of the general education classes are taught by part-timers, some want to create their own editions because the general editions are bland and don’t meet that department’s specific needs, etc.
In English, it is all about composition books. Though sometimes you get a little pre-packaged stuff with the literature anthologies, publishers know full-well that you can get a perfectly good new copy of about any classic work of literature for less than $10, so they don’t put too much effort in that area (who would pay an extra $30 to have the most recent edition of the same old translation of Homer?). For composition, on the other hand, we have one book rep or another roaming our department every couple of weeks, schmoozing the faculty and trying to buy free lunches for department and committee chairs. About once or twice per semester, some book rep wants to demonstrate some new service or product, and so they have a “demonstration” that is, of course, a sales pitch with lots of free food (by the way, in our department we always loot the food afterward, put it in the faculty lounge, and open the lounge to the starving students for the next day or two).
Oh, and, by the way, students in my composition class who wonder why I don't require them to buy the rhetoric -- now you know the reason. By requiring my students in Composition I to have only a copy of a handbood (a basic reference work everyone should have and keep anyway), Strunk & White's Elements of Style (available free online and used for under $5 everywhere), and the Bible (available free from all sorts of places), I can keep the costs for my class down to under $20, rather than approaching $100, as would happen if I assigned a standard rhetoric, reader, and handbook.
So, in short, why do they charge so much? Because that’s what the market will bear; you’ll pay nearly any amount if the professor requires it.