From the ALA: Library Value Calculator

The ALA and the Massachusetts Library Association have recently posted a Library Value Calculator online.
 
This tool is meant to help illustrate to patrons and the public how much money using their local library saves them and how to calculate the total Monthly and Yearly value of having access to that library.
 
Check it out: http://www.ala.org/advocacy/advleg/advocacyuniversity/toolkit/makingthecase/library_calculator