> it is still often true that you get what you pay for

Hasn't been my experience at all - in fact I haven't noticed a relationship between quality and price of textbooks in either direction.

> you spend so much time and effort seriously working through a math book that whatever money you paid for it looks like something that does not matter at all.

You'll spend the time if the book is good. That's an advantage of free books - you can discard the bad ones without wasting money.