Coin values are based on the base metal value (gold hit $1500/oz today), the condition, how many were minted, the strike quality, and rarity.
For example, a 1920-S 20$ gold Double Eagle has a minimum value of $1350 since the coin contains 90% gold. And if it's condition is a Mint State (MS) 65, it has a numismatic value of $325,000.00. Now, there were more minted in San Francisco (558,000) than Philadelphia (228,250). So why is the S worth 4 times that of the P? Rarity.
Look here for coin prices of US coins:
PCGS - Price Guide
NGC Coin Price Guide
DO NOT CLEAN THE COINS NO MATTER WHAT!!! That will subtract from its value. Have it cleaned professionally by one of the above. Not a coin dealer.
Keep looking at what you have.
I got this coin a couple years ago on ebay for $1500 ungraded. It isn't graded yet, but my best guess at grading make it a Very Fine 35 worth $6500.
