1943 copper penny; $10,000-$100,000+: Because of the war effort, most pennies were made out of steel. Some left over copper was accidentaly fed into the machine. 40 were struck, but only 12 have been confirmed to exist. To check to see if your penny is steel or copper put it up to a magnet. If it sticks it's steel. If it falls off, it's copper.
Silver Coins: The current price of silver on the market is about $38 per troy ounce. Coins were made out of 90% silver until 1964. From 1942-1945 nickels were made out of 35% silver. From 1965-1970 Kennedy Half Dollars were made of 40% silver. Also from 1971 to 1976 Eisenhower Dollar Coins were made of 40% silver (proof sets only)
Gold Coins: It isn't as likely that you will come across a gold coin as a silver coin. But if do happen to find one, these are the prices of gold today. Right now gold is worth about $1689 per troy ounce.
1944 steel penny: circulated-$40,000-$135,000+; uncirculated-$260,000-$346,000
1955 double die penny: just the numbers-1.00-50.00; the whole thing-300.00-25,000
1970 doubled die penny: 100.00-300.00