I think there could also be a psychological element, especially when seeking more expensive books.
It took me almost 10 years of collecting before I made the jump from spending hundreds on a book to spending over $1,000 on a book, and I'm going through the same psychological process now as I contemplate buying some of the massive keys I need to complete some of my runs...am I ready to make the jump to spending over $10,000 on a book? That's a lot of cash, but unless the comic market completely crashes and burns in the next 10-20 years you should be able to recoup most of your investment at worst, and make a nice profit at best.
Compared to other hobbies, comic collecting has been both fun and financially rewarding...