You have a British coin ("Georgius IV D.G. Britannia Rex F.D." is abbreviated latin for "George the Fourth, King of Britain and Defender of the Faith") dated 1821. To get a value, the denomination and condition of the coin would need to be known.
Britannia can be found on the reverse of a Britishnon-commemorative 50 pence piece. Britannia also appeared on predecimal Pennies and pre-1936 Halfpennies.
USD10k
Therre were no 1821 British Double Sovereign or Two Pound coins minted.
No US dollar coins were struck in 1821, look at the coin again and post new question.
No US silver dollars were made in 1821 look at the coin again, post new question
Please look at the coin again. No U.S. silver dollars exist dated 1821.
20 bucks
Other than for commemorative 50 Pence coins, Britannia has been on the reverse of the 50 Pence coin since it was first issued in 1969 until 2008 when the new design British coins were issued.
The US made NO silver dollars dated 1821, look at the coin again and post new question.
A denomination is needed, post new question
The U.S. did not mint any silver dollars in 1821. If you meant 1921, it's worth about $13 to $15 in average condition.If your coin is dated 1821, please check its denomination and post a new question. If it actually says it's a $1 coin it is a counterfeit.