Specifically, it is a US Army "greatcoat" button, manufactured between 1820 and 1839. For additional info and photos of the five known varieties (listed as buttons #GI-70A, 70B, 71, 71Bv, and 72), see pages 34-35 in the button-book by Alphaeus H. Albert. Your button appears to be the GI-70A version.
Kuger is correct, websearching is the best way to ascertain your excavated button's current retail dollar-value in today's relic market. In doing so, please keep in mind that the value of excavated ones and non-excavated ones is not the same. Also, I guarantee you that will find a rather wide price-range, because anybody can put whatever asking-price they want to on their website -- which often will have no connection at all to "reality" value in the market. A more accurate appraisal of dollar-value comes from "watching" the auctions on Ebay to see what the item actually winds up selling for, in several auctions, not just one or two.