On the other hand, your time is never free - to the point that no matter how much you pay for a math book, you will end up paying incomparably more with your time spent studying it.
This is true.
However, I find that I tend to buy much more than I read. Not just because I tend to hoarde these things, but also because I try to seek out sources that click with me before I work through them.
I take your 2nd para in a way, the cost of a book being so cheap when I do read up on something I tend to buy 2, sometimes 3, books on the subject so where I get stuck on one the other will see me through.
Still, I am discomforted by your book-buying habits; a book unread is a valuable resource wasted :) But each to their own way.
There are lots of fantastic books out there, and only so much time to read them. Sometimes I buy some with the intention of reading them later. But reading books properly takes time, so it's inevitable that I fall behind.
Also, lots of the techincal monographs I buy have some overlapping contents with others I own.
I can buy a book for a single chapter. That's the only way to get that chapter, so it justifies the purchase to me, even if the subjects covered overlap with other books I own.
“Unread Books Are More Valuable to Our Lives than Read Ones”:
https://www.brainpickings.org/2015/03/24/umberto-eco-antilib...
But it is also true that "freely available" is nto the same as "worth nothing" and some of the works here, at least, are well worth the study (to their intended audience), I believe.
But what if you are optimizing for enjoyment?