myotherhead
A textbook represents a distillation of the author's knowledge and experience in some specific field.
You may need that knowledge now, because at some time in the future you may be earning a living from what you know in that field.
Usually a business with some knowledge gap in their practices, hires a consultant for their knowledge in that specific field. By the same token, when you buy a textbook, you are "hiring" the author/s of a textbook as consultants, to fill in some gap or need in your knowledge. How much do you think you should pay your consultants(authors) for their knowledge and experience?
And that is not such a high price - I have read from some engineering contract law, and maritime law, textbooks that were in the order of $3500 each, if I had to pay for them. I have seen some specialist medical surgery textbooks in the same ballpark price.