Keep your eyes open for library book sales. My town library has one every year--I have slowly made some space on my shelves at home through donations--and I assume many of the surrounding towns do as well.

Alternatively I visit a book store every 5 or 7 years when we visit relatives. It's a big old barn of a used bookstore. I remember quite clearly in high school seeing a specific chess book, for $20! When I returned many years later it was still there. I will never, ever, learn all the opening moves to the italian defense but, i know the book is there if i ever need it.