What is the value of an 1820 US dime?

Two varieties were minted that year. You may need to look for pictures or have the coin inspected in person to tell them apart.

Numismedia lists the following approximate retail values as of 05/2010:

Large "0" in the date:

Very worn condition - $33

Moderately worn - $54

Slightly worn - $168

Almost no wear - $618

Uncirculated - $1,020 to $28,930 depending on quality

Small "0"

Very worn - $46

Moderately worn - $77

Slightly worn - $228

Almost no wear - $738

Uncirculated - $1,380 to $12,810