The "Mercury" coin is a dime. In addition the P mint mark was first used on $1 coins starting in 1979 so that letter can't be on a 1921 dollar.
A 1921 Morgan Dollar (eagle on reverse with spread wings facing forward) is the most common silver dollar and is valued by its silver content.
A 1921 P Peace Dollar (eagle perched in profile) is one of the harder dates to find and is worth at least $50 badly worn and a few hundred if uncirculated.
The intrinsic value is $14.16 with silver at $18.31 per ounce
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Assuming it's a more common Morgan dollar, about $13 for its silver content
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A average Mint State 1921 Morgan is $30.00-$40.00
The intrinsic value is $14.16 with silver at $18.31 per ounce
1922 silver dollar with closed eagle wings
A 1921 Half Dollar is 90% silver. It has a melt value of somewhere around $12. The related link below has more information.
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Assuming it's a more common Morgan dollar, about $13 for its silver content
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There is around .77 ounces in a 1921 US silver dollar.
A average Mint State 1921 Morgan is $30.00-$40.00
1921 is by far the most common date for Morgan dollars -- current value is about $17.00
There's a Peace dollar but not a war dollar. Please see the Related Question.
If the coin is a real Morgan or Peace dollar. The value is just for the silver, about $20.00.
There is no 1921 barber silver dollar, it is either a peace dollar (eagle perched on a rock engraved PEACE on the reverse) or a Morgan dollar (Eagle with gothic lettering above it saying "In God we Trust" on the reverse)