
Nice coin, Mark.
A US $2.50 gold piece contains 0.121 Troy ounces of gold. Multiply that by the current spot price of gold (you can find that number at
Kitco.com, it changes daily) then add $100 US dollars to get a ballpark retail value for your gold piece.
For instance, if the spot price is $960 US dollars per Troy ounce, then Mark's coin would be worth about 0.121 x 960 + 100 = $216. The $100 added value can be more if the coin is fully uncirculated.
DO NOT CLEAN YOUR COIN. CLEANING RUINS VALUE.
Just about all quarter eagles are very similar to Mark's coin in value. Here are some rules for finding more valuable coins:
1. 1841, 1854S, 1864, 1865, 1866, 1875, 1881, and 1885 are better dates. The generally are worth 1000s of dollars. Write to CoinQuest for a quote is you have one of these.
2. Coins with C (Charlotte) and D (Dahlonega) mint marks on the reverse under the eagle, are more valuable than coins with O (New Orleans), S (San Fransisco), and no (Philadelphia) mint marks. Figure about $1000 catalog value for C and D mint marks.
3. Coins dated before 1847 are a wee bit more valuable than later dates, even without a mint mark.
Few people know that we have three, not two, official coin denominations in the United States. There are 100 cents in a dollar, and there are 10 dollars in an eagle. So cents, dollars, and eagles are our three monetary denominations. You have a 2 1/2 dollar, quarter eagle, gold piece.
The value of your quarter eagle depends directly on its condition, or numismatic grade. If the coin is truly uncirculated, it is worth much more than circulated examples. The presence or absence of luster is the best way to tell if a coin is approaching uncirculated state. The coin in the picture has some luster near the outside, that is, near the rim. But the luster has been worn off toward the center. This coin is in Almost Uncirculated (AU) condition.
Luster is is best described as 'creamy' rather than 'shiney' or 'polished.' In fact, shining or polishing a coin destroys its numismatic value.
If you think you might have a truly uncirculated coin, feel free to start an e-mail exchange with CoinQuest using the Contact Us link at the upper left. Via e-mail you can send us a picture of the coin and we can give you a better appraisal.

As with all valuable coins, you must recognize that counterfeits exist. This picture shows a fake US $1 gold piece. It is an obvious replica, not the real thing. Many counterfeits are not obvious at all.
We have a few other specialized appraisal pages that address these coins, here:
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A damaged quarter eagle
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Special quarter eagles 1859 to 1861