I definitely borrow more than i buy. A) because I can't afford everything on my amazon wish list and b) I'm a minimalist. I hate having things around that will just collect dust.
On the rare occasion that I do decide to purchase a book I do so because my local library does not carry whatever title it is i'm looking for. Usually when I'm done with the book I donate it to my library. I do this with movies as well.
The last book I bought was How To Die In Paris by Naturi Thomas for $3.71 on eBay!!! It's already in the library pile.