For example, when I took thermodynamics in college (in 2001), I used a thermo book from the early 70's which cost me $8 on eBay (as opposed to the modern version which cost $100). I also skipped purchasing many books entirely, and just used free materials online, including this newfangled site called wikipedia.
Agency costs and are the issue. The people assigning books don't have to pay the cost of using them, so they have no incentive to select based on price. Further, it's very difficult for students to use substitutes - if the professor assigns problems 1-10 from the book, it doesn't matter how good the material in your cheaper substitute is.
If you want to crack this nut, focus on problem sets, not textbooks.