The Denver Public library has a Library Value Calculator on their website. It was mentioned in this story about a person who decided not to buy any books for one year. (Article indicates they are a librarian)
Excerpt: There are several reasons I stopped buying books in 2008. With a young child at home, a car payment and student loans, saving money was becoming more important to me than owning "Zazie in the Metro" or "Tamerlane: Sword of Islam." As a librarian I also saw a limit on book buying as an opportunity to enrich my professional life by experiencing the library more fully as a patron. Finally, part of me just wanted to see if I could do it.
In the comments to the article there is this comment: Thank you Denver Post, for printing an article that damages further an already damaged industry, the local book shop. While libraries have their places, we desperately need the few surviving bookshops. How about an article boosting the buying of books?