Price guides are helpful to a degree, but I think we all can agree that a single price for a Babe Ruth signed photo, for example, is a bit too simplistic when they can range over thousands or even occasionally tens of thousands of dollars based upon size, condition, personalization/notation, etc. Though it's admittedly a bit self-serving, I can honestly identify the Heritage Auctions "Permanent Auction Archives" as one of the most helpful online sources for valuation. Every lot we've ever sold (over 55,000 to date) is available with full images, auction copy and prices realized. And it's 100% free to join and view.
http://sports.ha.com/?tab=archive