It's a mintmark, showing which mint made the coin. An S is the San Francisco mint a D is Denver. The P is for Philadelphia and is only on coins made from 1980 to date except the penny it has never had a P mintmark.
The value would depend on the condition and date of the coin.
a date
The mint is the place where the coin was made. In the US there are several. If the coin has a D next to the date, it was minted in Denver. S is for San Francisco.
The coin was made in 2007 NOT the 1700's. The date 2007 is on the edge of the coin. Also the coin is NOT gold it's brass and is only one dollar.
Unless there is an "S" under the date, it is a fairly common coin that sells for 10-70 cents depending on the condition. if there is an "S" under the date it is a more desirable coin worth a few bucks in very fine or better condition.
It means that the coin is rare. For example a 1909-S VDB cent is a key date. This coin is rare.
All US Quarters made in 1975 & 1976 have the dual date 1776-1976 so if the coin has a date of 1975 it's not a US coin.
Much more information is needed - please post a new question with the coin's date and the country that minted it. If it's a US coin, look for a small D or S mint mark.
If I understand the question, you have a Sacagawea dollar coin with no date on the obverse (front) of the coin and you think it should have a 2000 date on it? The coin was made after 2008. From 2009 to date, the year the coin was struck in is on the edge of the coin.
US one dollar gold coins were last made 1889. Make sure you used the correct date. If your coin has a different date ask another question like the one above but substitute the date with the date on your coin.
On the "heads" side of US coinage there is a date, that tells you when the coin was made.
In 1960 I bought a US 20 dollar gold coin that is encase in a gold frame to be worn as jewelry. This coin is 1924 and was minted in San Francisco I assume as it has an s by the date. I would like to know what is the value of this coin.