According to InvestmentNews, a U.S. 1943 Lincoln head copper penny in flawless condition is worth $1.7 million. Only 40 of these coins are known to be in existence.
In 1942, copper was being heavily rationed by the U.S. government for the use of ammunition. Pennies had previously been made from copper, but because of the rationing, the mint switched to using steel. A few of the copper coins found their way into circulation, however, and they are now some of the most rare and valuable coins of the modern era. They are composed of 95 percent copper and 5 percent tin and zinc.