John: You have a reproduction of a famous Comitia Americana medal originally issued for Nathaniel Greene, major general of the Continental Army in the American Revolutionary War, and known for his successful command in the Southern Campaign. He forced British general Charles Cornwallis to abandon the Carolinas and head for Virginia. After the war Greene emerged as George Washington's most gifted and dependable officer, receiving this medal from the Continental Congress. Bronze, silver, and gold versions of the medal exist. All are extremely valuable.
As with many valuable numismatic items, legal reproductions and illegal forgeries often are produced. The pewter example shown in our secondary picture (and marked 'Reproduction') sells for $10 US dollars or so on the retail market. A coin dealer might offer $2 to $4 for it. If you are not sure what your have, seek out a professional coin dealer or a knowledgeable collector for an in-person appraisal.
The bronze medal in our main picture (upper left) comes from Stack's Bowers, where it was auctioned in 2011 with accompanying congressional documents for $86,250. As always, CoinQuest is grateful to Stack's Bowers for use of their coin photo. Be sure to check Stack's Bowers page on these medals.