I just use this... I don't know is its greysheet prices but it's a pretty nice online one that updates the prices often. http://www.numismedia.com/rarecoinprices/fmv.shtml
There are two free consumer (retail) price guides that I know of - Numismedia above, either coinprices.com /or/ coinprices.org (one of those is the consumer version of the greysheet which is paywalled - I just don't remember which is which...) The old NumisMaster (F+W Media / Krause --> AIM in the bankruptcy auction) is behind a paywall.