That's too broad a question to have a single answer.
Modern gold bullion coins sold as investments are worth the amount of gold in them, based on the current price of gold on the open market. (These coins' denominations are artificial.)
Old gold coins (1932 and earlier) are worth at least as much as the gold they contain (usually many, many times their original face value) but can be worth even more as collectibles. You have to know what country issued the coin, what its date is, and whether it has a mint mark.
Modern "golden" $1 coins from the U.S. and Canada don't contain any gold, they're brass and are only worth $1 each, despite many popular misconceptions.
A British Gold Sovereign is struck from 22ct gold and weighs 7.98 grams.
Based on the current gold price of $1,373.25 / oz, a Sovereign is currently worth $322.73 / £201.90.
The price of gold (gold fix) changes twice a day
Depends on what gold coins you are talking about. There are 2 broad categories of gold coins, bullion coins (Krugerrands, Gold Eagles, Gold Maple Leafs) and numismatic gold coins (British Guineas, early US gold, modern US commemoratives)
Gold is around $1,600 per troy ounce and that is the value for bullion coins. A 1/10 troy ounce Gold Eagle would sell for $160 and a full gold eagle would sell for $1600.
Numismatic gold is trickier, there are some that sell for very close to bullion prices (Liberty head double eagles for example) but there are some that exceed bullion prices greatly (for example, a common date double eagle might sell for $1700, a small premium or may sell as high as $7 million such as the 1933 double eagle)
Gold plating, however, adds no value to a coin and serious collectors will view the coin as damaged.
The US never made pure gold coins. They were always 90% gold and 10% copper. Post a new question and include date denomination and mint-mark if it has one.
$1133.30 right now. Just a few months ago it was about $300. It may be different in an hour. You can get the current market price at www.quotegold.com
A denomination and date is needed, post new question.
NOTE: The only "Pure Silver" coins made by the U.S. are American Silver Eagle 1 Ounce Bullion coins starting in 1986.
1200
Sorry no US gold coins dated 1917 were struck
Sorry, no US $10.00 gold coins exist dated 1922
None of the modern US gold bullion coins are worth millions.
Sorry, the US produced no gold one dollar coins in 1933. Post new question.
The US did not produce any gold coins for circulation after 1932 and no dollar coins were struck in 1956. Look at the coin again and post new question.
Sorry no US $10 dollar coins date 1916 were struck.
Galleons are coins of the Wizard world so their exchange rate is unknown to us Muggles.
The only one ounce gold coins made by the U.S. are the American Eagle gold BULLION coins starting in 1986. Post new question with the denomination of the coin.
It's plated. The US never made gold dimes. No gold coins of less than one dollar have ever been made.
8-5-11>>> Every gold coin made by the US has the word LIBERTY on it. A denomination is needed, post new question.
$0.61 - good luck on those gold buffalo coins you bought being worth anything.... sincerely, your government -----[Clarification on this answer - Actual gold buffalo coins (also known as American buffalo) minted by the Federal Government are solid gold, and as legal tender worth $50.00 US but are far more valuable due to their gold content. The coins inferred above are made by private companies, are only gold clad/plated with 14mg of gold worth under $1.00 US, and are not legal tender. These coins should have the word COPY on them, and retail for under $20.00 US]
No US one dollar gold coins are dated 1890, post new question