Textbooks are made specifically for each curriculum, and as such, there is not a gigantic market for them. As well, they are heavily information-based, and written with the intent of teaching, so they are not easy to write.
So, to pay the people who write the texts, as well as to pay the printing fees, and to post a profit, they have to make the textbooks expensive.
As well, if they're more expensive, then people are less likely to lose them.
Also, technically you don't need to bring in 90 bucks and the textbook. The money is to replace the textbook. Also, I've never heard of not being allowed in class when you haven't returned your texts, where I attend, you just can't get any new textbooks.
Get that book in. ^_^
At least you aren't in university, where you have to PURCHASE all of your texts. And their prices aren't any cheaper.