There is a thorough web page on the Great Seal of the United States at Ask.com. The medallion in our picture is the one you see the most. It is a replica of the seal commissioned in 1882, the 100th anniversary of the seal's adoption. Other versions, like the one owned by Chris, are also around, but not as plentiful.
These medals sell for $10 to $50 US dollars on eBay, depending on condition. With a retail price in this range, figure a dealer will buy one from you for one-half retail. The dealer uses the mark-up to keep the business afloat.
Novus Ordo Seclorum means, roughly, New World Order and Annuit Coeptis can be translated to God favors our undertakings. There are all sorts of spooky theories about ties between the Great Seal and Free Masonry. A search on the Internet will yield many interesting, but inconclusive, results.