What is the value of a 1921 silver eagle?

This question comes up fairly often and is a common misunderstanding. US $1 coins made up till 1935 aren't called "eagles" and they don't contain 1 oz of silver. They were ordinary circulation coins made for use in daily commerce. While they do contain silver, it's not as much - about 3/4 of a troy ounce.

"Silver eagles" are modern (1986 and later) coins sold by the Mint as investment pieces, not for spending; their value is linked to the current price of silver and the "$1" value is purely artificial.

The coin you have was worth exactly $1 when minted because silver was much cheaper and its price was controlled. Today it has value not only for its silver content but for its interest to collectors. There's more information at the Related Question.