Your 1927 Double Eagle was minted in Philadelphia. The Philadelphia mint used no mint mark prior to 1942. In 1942 the letter "P" appeared on the reverse [tails] side the Jefferson Nickel. It was used during the war years of late 1942 through 1945 when nickels contained 35% silver and no actual nickel. After 1945 the Philadelphia mint again used no mint mark for the coins produced there until 1979. Again, the letter "P" appeared but this time it was on the new Susan B. Anthony Dollar. In 1980 it was decided the letter "P" should appear on all coins minted at Philadelphia except for the Lincoln Cent.
About $1000 regardless of mint mark, due to its gold content.
In 1927 the US Mint at Denver, Colorado struck 180,000 Double Eagles. There is no way to know how many of them still exist today.
About $1000 for its gold content, regardless of mint mark.
From about $1300 to $19,000 depending upon the mint mark and the condition of the coin.
New Orleans mint mark is "O".
$1300 to about $150,000 depending upon the mint mark and the condition of the coin.
The 1928 Saint-Gaudens double eagle was only struck at the Philadelphia Mint so no coins bear a mint mark. But for coins of this series that do have mint marks it's just above the date. All so the 1928 double eagle is the highest mintage (8,816,000) of any US gold coin of any type.
A mint mark tells where the coin was minted, for example, a P mint mark (or in some cases no mint mark) signifies the coin was minted in Philadelphia, D is for Denver, S is for San Fransisco. I'm assuming someone told you that the mint mark was under the eagle for whatever coin you had, so you simply need to look under the eagle to find a mint mark. Keep in mind that some coins were produced in the Philadelphia mint and have no mint mark.
What date? What condition? What mint mark? Please post a new question with those details.
The mint mark is near the bottom center of the side with the eagle. There might not be a mark.
The mint mark is located on the eagle side of the coin and is between the word " fine " and the eagle.
The first Half dollar with any mint mark was from the New Orleans Mint in 1838.The mint mark is on the reverse under the eagle