According to Official Guinness Records,
The most expensive coin in the world is the 1933 Double Eagle, a $20-gold coin which was auctioned at Sotheby's, New York, USA on 30 July 2002 and fetched $7,590,020 (£4,856,370) including buyer's premium.
As reported in The New York Times 31/7/2002: 'In 1792 it was decided that all gold and silver United States coins would bear the depiction of an eagle, but gold pieces were issued only after the Gold Rush Boom, in 1850. They were called double eagles because their face value was twice that of the original $10 gold piece. Following the suggestion of President Theodore Roosevelt, the sculptor Augustus Saint-Gaudens redesigned the coin in 1907 in high-relief, forever after giving the coins the designation 'saints'. In 1933 President Franklin D. Roosevelt took the US off the gold standard, and ordered all 1933 saints already manufactured to be destroyed, save for two reserved for the Smithsonian Institution. They were never declared legal currency. Presumed stolen by a mint employee, nine double eagles surfaced in the 1940-50s. They were seized by the Secret Service and melted down. But before these coins came to light, the royal legation of Egypt turned up with one at the Department of the Treasury and somehow obtained an export license for the coin to enhance the collection of King Farouk. By the time the mistake was uncovered the coin had already left the country. After the king was deposed in 1954, his coins were sold at auction and the double eagle disappeared. It went underground until Stephen Fenton was arrested in Manhattan trying to sell the coin for $1.5 million. The double eagle was moved from the Treasury Department vault in 7 World Trade Centre eight months before September 11 attacks. The coin is the only 1933 double eagle ever to be legally issued by the US government. The coin was 'monetized' after the auction making it legal tender. The buyer will receive a certificate of transfer stating just that, only after paying the auction price and a fee of $20 for the face value of the coin.'
For a complete list of 2002 records, please visit 2002 Guinness Records in United States.