LAS VEGAS (AP) — The coin trading world has a new gold standard with the $8.4 million sale at auction of the only known 1822 half eagle $5 piece in private hands. Coin experts confirmed the coin that traded hands Thursday in Las Vegas is one of three of its kind in existence and the price was the highest for a U.S. gold coin struck by the U.S. Mint. The buyer remained anonymous. The price approaches the most-ever for a U.S. coin, after a 1794 U.S. “Flowing Hair” silver dollar that once fetched $10 million. Thursday’s public auction was staged by Stack’s Bowers Galleries.