LAS VEGAS -- The coin trading world has a new gold standard, after the only known 1822 half eagle $5 piece in private hands sold at auction in Las Vegas for $8.4 million, experts said Friday.Douglas Mudd, curator and director of the Edward C. Rochette Money Museum in Colorado Springs, Colorado, confirmed the coin is one of three of its kind in existence and the auction price Thursday was the highest for a U.S.