Science textbooks generally are more expensive because there's actual useful information contained in them. I paid $150 - $200 for my engineering textbooks and still have (and use) them. Compare that to $100 for some textbook on Comparative Hungarian Furniture and I consider it a deal.
I know what you mean. Having graduated last year, I have experienced this myself. Don't forget the professors who use a book that will be replaced the next semester. Of course, you pay full price for it because there are no used copies available. When you turn it in, you are lucky to get any money whatsoever because it was discontinued. Then there are the buyback policies that stink. When they buy the books back, you take such a loss that it is pathetic. Often times I would sell the book at the amount that the bookstore would offer to another student, just to be a stinker. Or I would give other books away rather then let the highway robbers have the business. Petty, I know, but it felt good.