This isn't a textbook though. Well, actually, I'm not sure what you mean by textbook. I think "textbook" implies little or no primary research and a compilation of summaries, case studies, study questions and so on meant for a reading audience comprising students and their teachers. Usually large publishing houses produce and distribute them according to contracts with academic institutions. There are typically multiple authors and the overall vibe these works give is impersonal.
(Ok, I know there is a looser use of the word "textbook" which is used to mean any work used in a classroom, and some academics produce original works which even they will call "textbooks" because the purpose is to help students and guide teachers through a difficult subject.)
Kolbrek and Dunker's book is academic literature (in the same sense of the word used for novels). It is an original work that took years to put together, and has unique historical and technical information that can be cited again and again by pros in the field, engineers and researchers alike. Publication was likely arduous. In fact this book was self-published, and all of the typesetting, layout, registration, and I would guess a lot of the legal work, was done by the authors. A third party printed it. And the authors likely keep track of stock, and negotiate with certain sellers for distribution.
I won't go into what I've paid for books, even as a student, and I'm not a bibliophile whose interest is down to particular errata or special binding materials and paper (bibliophile talk is split between subject matter, historical significance and "feel" on the hand, and it's a much older pastime than ours, so you can imagine the bibliophoolishness around). If there is a principal at stake here, it's about the accessibility of knowledge. I'd argue that this book is way more valuable than any textbook, and that textbooks themselves hinder knowledge more than help it.
Maybe this might sway your perspective just a little. Not to buy. Not even to say the price is justified. But to consider the different situations at play here.
) a contributer to the late and lamented Art Dudley's late and lamented Listener magazine. 
