Rodney, everyone else is right that the value depends on the customer. However, a Gravely front mount cart is probably one of the rarest and most desirable attachments for a Gravely collector.
I can't tell you an exact dollar value, but I would expect to easily sell it for a couple hundred if the right collector had a checkbook in his hand. Better yet, put a $200 reserve on it and sell it on Ebay after advertising it on the Gravely forums. Let those with checkbooks argue the value and run you up a nice price!
Ben W. in VA