That's a tough but great question. I spend a lot of money on books very loosely. It's one of the areas of my life where I don't really think about a budget. I just want to consume more and more if I can. And I leverage my local library, local bookstore and Amazon to the hilt.
I don't remember who said it but I take the advice of "if there's even a single thing in the book that you make note of, it was worth buying". And there's a lot more than a single thing. I actually skimmed through the book a second time and jotted down notes from each chapter I want to dig deeper in (e.g. another author mentioned, a research paper, one of the exercises).
I remember hearing a similar thing from a guy that was sitting in for Tim Ferris on the podcast. He said in anything you read, try to take away one important thing.
I don't remember who said it but I take the advice of "if there's even a single thing in the book that you make note of, it was worth buying". And there's a lot more than a single thing. I actually skimmed through the book a second time and jotted down notes from each chapter I want to dig deeper in (e.g. another author mentioned, a research paper, one of the exercises).
So for sure I think it's worth buying.