How much is a dollar coin worth?

If you mean a modern (2000 or later) "golden" dollar or a Susan B. Anthony dollar, it's worth face value only. Feel free to spend it.

Eisenhower dollars are only worth face value if made of copper-nickel but they may not be accepted in many stores due to their size.

Any true silver dollar (1935 or earlier) is worth much more than $1 for its silver content, and possibly even more than that as a collectible but a date, mint mark, and condition are needed to make an estimate.

Slightly, intangibly less than a dollar.

The US population has amply demonstrated they don't like dollar coins, and on the whole prefer paper money.
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