$40 for a .pdf with large font and only 90 pages of content is not unusual at all. Neither is stuff in the $70 range. The high price is due to the supposed "value" of the information.
I've actually always thought those e-books were totally overpriced, but I guess a lot of people had no problem buying them.
It's weird though how the set point for price is expected to be much lower for more traditional books that have been ported over, even though in many, many cases the real life books offer tons more value than a cheaply thrown together e-guide to parrots or dating or something.