PGA.com Value Guide, the National Standard For Golf Club Values and Information
I believe many of the major golf stores (Golf Galaxy does) uses this guide for trade-in values when you want to upgrade, but the prices there are low IMO.
If you're an eBayer, do a search for completed listings and see what price your model clubs sold for in the past 30 days or so. That's what I've done when pricing clubs I sell there or on CraigsList.