Sassymonkey Reads had a great post last month about how much money the library saved her in 2009. She figured that if she had purchased all the books she read throughout the year, she would have spent $1,239.27 - enough for a fine long vacation or a few smaller weekend jaunts! She also addresses the old "But the library charges me fines and so I lose money" whine with some eye-opening stats that any of us can arm ourselves with.
It reminds me of a week-long marketing campaign during National Library Week that we did at a small-ish library I worked at a few years ago. Our circulation system let us easily see, at the time of check-out, what the dollar value was for the material checked out. We purchased big price tags, pre-stickered them with a note to the effect of "What is your Library worth to you? You just checked out $___ worth of materials. Happy Library Week!" Then we filled in the total.
It was a fun and eye-opening week for our public. Parents who always checked out bookcrates of books couldn't believe they had $950 worth of good stuff. Adults who took out an armful monthly were pleasantly surprised that they had saved $125 each time. It helped us alter the perception of the public and put a dollar and cents savings cost to them. We were also able to chat and tell them what a $120K household (about median house price at the time) paid in average taxes for the library annually (way under $100). It was a one-two punch and SCORE! for our public and a simple way to show the worth of using libraries.