Most people don't know it, but there are three American coin denominations: cents, dollars, and eagles. There are 100 cents in a dollar, and 10 dollars in an eagle. You have a half eagle, or $5 gold piece.
The value of these coins goes up and down with the gold market. Each coin contains:
QUARTER EAGLE 2 1/2 DOLLARS: 0.121 troy ounces gold
HALF EAGLE 5 DOLLARS: 0.242 ounces gold
A good way to estimate the value of these coins in a changing gold market is to multiply the current gold price (you can use kitco.com to find the current gold price) by the gold content. This gives the base value (BV) of the coin, sometimes called the melt value, or the value if you melted the coin into a puddle of gold. For gold at $900 per ounce, the melt value of a half eagle is 0.242 times $900 = $218. Look up the price of gold now. It changes every day.
Once you have the base value (BV) of your coin, add more value to account for numismatic (coin collector) appeal.
worn: BV + $20 US dollars
average circulated: BV + $50
well preserved: BV + $100
fully uncirculated: BV + $250
worn: BV + $50 US dollars
average circulated: BV + $100
well preserved: BV + $200
fully uncirculated: BV + $400
This will give an approximate retail price for coins with common dates. If you sell such a coin to an honest dealer, figure he or she will pay about 60% to 80% of retail price. The markup is necessary to keep the dealership solvent.
Just about all the dates are worth about the same, but there are some noted exceptions. The lists below show approximate catalog values for coins in well preserved condition.
1911D: $4000 well preserved
As with all valuable coins, you must be aware that counterfeits exist. This is easily true with these series, where a crook can take a normal coin and add an mint mark with a few jeweler's tools. See this page for more info on fakes.