Look on the back of the coin to see if there's a small mint mark letter below the eagle. It may be blank or there may be a CC or S.
Numismedia lists the following approximate retail values as of 05/2011:
No mint mark (Philadelphia):
Very worn condition - $140
Moderately worn - $258
Slightly worn - $294
Almost no wear - $618
Uncirculated - $840 to $31,850 depending on quality
"CC" mint mark (Carson City):
Very worn - $264
Moderately worn - $510
Slightly worn - $670
Almost no wear - $1,256
Uncirculated - $1,770 to $87,750
"S" mint mark (San Francisco):
Very worn - $94
Moderately worn - $144
Slightly worn - $174
Almost no wear - $382
Uncirculated - $640 to $24,380
DISCLAIMER: The retail values quoted are the best available as of the date shown, but may vary significantly due to changes in the precious metals market. Also the wholesale (buying) price of a coin will be less than the selling (retail) price. A reputable coin dealer will be able to give a more accurate valuation based on inspection of the coin.
Check that coin again. The only years for the US 20 cent coin was 1875-78.
The US only issued 20¢ coins from 1875 to 1877. If you mean 50 cent piece, please see the Related Question.
20 cent coins were only made for four years: 1875-1878
Sorry no US 20 cent coins dated 1802
The only 20 cent US coins were minted 1875 - 1878. You have mis-read either the date or the denomination.
All twenty-cent pieces are scarce and valuable. However, you need to provide the mint-mark and some gauge of the condition for anyone to provide you with an accurate value. The mint-mark is very important, the 1875 S 20 cent piece is the most common of all of them with over a million minted, however the 1875 CC 20 cent piece is even rarer with only a bit more than a 100,000 minted, and the 1875 20 cent piece with no mintmark is rarer still with less than 40,000 minted. The mintmark will be above the Y in TWENTY on the reverse of the coin. Do not clean your coin. Cleaning a coin can only cause it to lose value, sometimes dramatically. 20 cent pieces are one of the most valuable denominations of US coinage to collectors. It is worth far more than its silver content even if it is in poor condition and a 1875 S, however, without knowing the condition and mint-marks of your coin, I can't say how much it is worth because values change dramatically depending on condition and mint-mark.
Rm3000
A U.S. copper 1826 Coronet cent is valued from $20 up to hundreds of dollars depending on wear.
A 1962 half dollar is 90% silver. The circulated value is $7.01 because of the silver content. The uncirculated value is $20.
well-worn = $10 lightly worn = $20
A Zimbabwe 20 cent coin will be recent, and almost worthless. Keep it for its sentimental value.
There was no Royal Wedding 20 cent coin issued.