Consider that some technical books have a potential target audience of thousand or so people. Then ask how many hours the book needs to save you to be worth $100. Depending on the book that can look very cheap.
Oh I forgot that's a thing in some countries. I did both of my university degrees just borrowing books needed from the library, they have to have enough copies for every student if needed.
Most of the time we'd end up with a group of friends where we'd each buy a different book then share among each other. Not many were spending the whole amount every semester, especially past the first year when we didn't really know any better.
I think I paid similar for Wolfram's A New Kind of Science back in the early 00's. Huge great beast of a hardback tome, promising the secrets of the universe. I don't think I ever finished it, and ended up giving it away to a charity shop after schlepping it around the world with me, unread.