Textbooks are expensive. Students know this and professors know this.
At the institution where I teach, we use the same textbook for two different Earth science courses. One course covers weather, climate, and oceanography and the other course covers geology. Both are designed as State University of New York (SUNY) general education courses in natural science for non-science majors (in other words, students majoring in, for example, art, are required to take a natural science course as part of their liberal arts education and either of these courses work).
The text we use is a popular book that's been around a long time (13th edition now) and, in my opinion, is well-written and comprehensive (full disclosure - Prentice Hall paid me to be a content reviewer of a recent edition of the text).
At our college bookstore (Folletts), this book sells for $163.25 new, $122.50 used, and there are rental options for new, used, and digital versions of this book (the digital rental is the cheapest at $76.75 for 180 days).
Amazon, on the other hand, charges $118.49 for a new edition of Earth Science and you can get free shipping. The Kindle edition is $100.94 and doesn't expire in 180 days. If you buy from 3rd parties through Amazon, you can get new versions as cheap as $88.00 plus $3.99 shipping and used versions for even less. It definitely pays to get your textbook's ISBN from a professor early and order online.
I also place a version of the textbook on closed reserve in the library since we do have students who are simply too poor to purchase a textbook (the publishers and the bookstore hate this practice!).
Many students, of course, will opt to buy textbooks from somewhere other than the college bookstore (often from a student who took the course a previous semester) or simply opt to not purchase the text altogether (although, in my experience, students who don't read the text usually don't do as well in the course). The college bookstore responds by not ordering a full complement of books for the course enrollment and then some students who wait until the last minute (usually for financial reasons - they can't buy a text until they get a financial aid check) can't even buy a text the first week of classes because it's too late to order online and the bookstore's sold out.
It's a mess. At my institution this past fall we had chemistry students without a text three weeks into the semester due to the bookstore being unable to obtain more copies (chemistry students use their textbooks for problem solving assignments just like math students do - not having a text is a disaster!).
Speaking of chemistry, textbook costs are even worse for science majors - many lab courses in the sciences require both a text for lecture and a lab manual for lab (and lab manuals in geology run from $75-$100 and may not have used versions available because student write in lab manuals). In courses like chemistry, physics, and calculus, textbooks may be several hundred dollars in cost (although they may also be used for two or more semesters).
So why are textbooks so expensive anyway? Geology textbook author Donald Prothero wrote an article on this very topic (Why are textbooks so expensive?). It's complicated, publishers are the ones making the bulk of the profit, but the bottom line is that it's expensive to produce a book loaded with color illustrations and photos that needs a lot of peer review.
Many professors eschew textbooks altogether but I haven't done that yet. Geology is such a broad subject, and so visual, that textbooks do help. I simply can't cover all I need to cover is 42 hours of classes for a typical semester - students need to do that extra reading to flesh out the stuff I cover more superficially in lecture. Many of them, of course, don't realize that, at least until the end of the semester when they see the final exam!
By the way, I'm always surpised when I see students selling back all their textbooks. I still have my original physics text, chemistry text, calculus and differential equations texts, upper-level geology texts, etc. While some topics do get out of date (introductory geology texts are different now than they were 30 years ago, but Newtonian mechanics hasn't changed). I love textbooks and have tens of thousands of dollars worth in my office (of course, as professor, I get many for free now) and refer to them surprisingly often!