I'm always trying to keep my students' textbook prices down. Here are some of the lower-priced books I've found. I'd be glad to hear any other suggestions any other philosophy instructors have found helpful.
For the ancient and medieval historical intro class that I've taught a number of times, there have been two books that I've liked. I had settled into Julia Annas' Voices of Ancient
Philosophy at one point, since it organized the material by topics (which is arguably better suited for an intro class in some ways than working through the material chronologically, which admittedly does have other advantages), and I love a number of her more idiosyncratic choices of texts. Amazon sells it for $52, though, and I still had to provide some medieval sources. The college bookstore always jacks the price up noticeably above list price, too. I've used Penguin's edition of Augustine's City of God, and I've tried a few different Aquinas anthologies, one from Oxford World Classics and the other from Hackett (Aquinas: A Summary of Philosophy). Along the way, I discovered Nicholas Smith's (et.al.) Ancient Philosophy: Essential Readings with Commentary, which contains a pretty large amount of material for only $35.
I should say that the best inexpensive texts for historical sources are from Hackett, Penguin, and Oxford World Classics. The two things I look for are readability (at least in intro courses) and whether they include marginal page numbers and such markers, since some of the texts for ancient and medieval sources don't, and it's much harder to find a passage if you don't have those. I've looked at Amazon's preview function to compare translations for a number of these books. Sometimes one translation is much harder to introductory students to grasp.
For early modern texts, I usually use Jonathan Bennett's online translations. Those are free, and they're much more readable than anything you can buy. For an advanced history of philosophy class, I might hesitate to use these, although I'd probably do it for a 300-level survey. I don't hesitate at all with intro courses.
Other books I've used include Greg Ganssle's Thinking About God, which is an excellent introduction to philosophy of religion. It's the most readable introductory book I've ever seen. It's fun and funny. But it seriously looks at the issues, and while I don't agree with Ganssle on every point I think he's especially fair on some pretty controversial questions.
Ted Sider and Earl Conner have put together an introductory-level metaphysics book called Riddles of Existence. I think Ted Sider's chapters are better-suited to an introductory class. Conee's are generally harder and often on more obscure topics. In a few places in Conee's God chapter, I found myself wondering if he'd even looked at the literature on these questions, since the objections he were presenting were not just easily handled but known to have been dealt with by those familiar with the philosophy of religion literature. (This is a disturbingly-common trend among specialists in other fields who throw philosophy of religion into their intro works on more general topics. James Rachels had the same problem with divine command theory and natural law theory in his intro to ethics, which I've nonetheless used a number of times. The new editions edited by Stuart Rachels have improved in some ways but not at all in that aspect.)
Speaking of ethics, I have trouble using the Rachels book that I previously liked to use. It's gotten too expensive without getting any longer. I remember when it was $35 for the same content, and I was shocked to discover a few years later (after ordering it for my class) that it had jumped to about $50. I don't think I've used it since then. Now it's more like $70. It's not much longer than the Sider/Conee book, but the price difference is huge. For ethical theory, my favorite book that costs very little is an anthology edited by Louis Pojman for Hackett. Last I knew, it was about $20 for a book most publishers would probably charge at least $50 for. The title is Moral Philosophy: A Reader.
I haven't had a chance to teach applied ethics inexpensively except when I've picked a couple topics and ordered books focusing on those. The typical anthologies are far too much money for me to want to have students use them, but sometimes I've decided that it's easier to use one huge textbook than to have them buy several smaller books on other topics, which could add up to too much if I want sufficient variety of topics.
Prometheus Books has a cheap but fairly comprehensive anthology on abortion edited by Baird and Rosenkrantz. It's not as good as the similar volume edited by Pojman and Francis Beckwith, but the price difference is large enough that I'd use the Prometheus. Prometheus also has a low-priced anthology of articles on the philosophy of sex and love. There are a few volumes you can get on that topic, but theirs costs the least. I've occasionally used other books that don't cost too much, but there aren't any that stand out in my mind as particularly compelling for repeated use. I did recently come across two low-priced anthologies that I haven't had a chance to look at, but I might consider them for future classes. One is Laurence Thomas' Contemporary Debates in Social Philosophy, and the other is Andrew Cohen and Christopher Wellman's Contemporary Debates in Applied Ethics. I'm curious if anyone has had a chance to look at these and offer advice about their suitability for an intro ethics class or a 300-level applied ethics class.
One other source that I like is Hackett's dialogues. They are especially helpful in an introductory class. My first philosophy class as an undergrad used the free will one by Clifford Williams, and I've used that in my own teaching. The two that I most use are Jay Rosenberg's Three Conversations on Knowing and John Perry's on personal identity and
immortality. I haven't spent any time in their others, but I know there's also one by Perry on the problem of evil (or maybe on theistic arguments for and against) and one by Rocco Gennaro on philosophy of mind.