We had a maths lecturer who got us to buy their text book. Turns out the text book was just copy pasted sections from other maths textbooks in one book. Cost 30 quid, had the old page numbers and was therefore a confusing mess throughout.
My tutor then pointed me to a better maths textbook which was actually worth the money, shame I didn't know about that before!
There's a very good book called E=mc^2: A Biography of the World's Most Famous Equation written by David Bodanis. I think I read it at about 16 and it was very approachable.
> Add on to this the customer support, marketing, warranties, legal expenses, returns and other typical overhead of production.
This is often why startups and SMEs can sell at lower cost too. The overheads are lower. I work for a medical devices company and our main competitor is more expensive than us and keeps having layoffs (or so it is rumoured). It's hard for them to cut prices for similar products when they have 10x the staff.
My tutor then pointed me to a better maths textbook which was actually worth the money, shame I didn't know about that before!