What is the value of a 1911 US 20 dollar gold piece?

Look above the date to see if there's a small mint mark letter. It may be blank or there may be a D or S.

Numismedia lists the following approximate retail values as of 06/2010:

No mint mark (Philadelphia):

Very worn condition - $1,500

Moderately worn - $1,570

Slightly worn - $1,680

Almost no wear - $1,728

Uncirculated - $1,800 to $55,250 depending on quality

"D" mint mark (Denver):

Very worn - $1,500

Moderately worn - $1,570

Slightly worn - $1,680

Almost no wear - $1,728

Uncirculated - $1,800 to $18,920

"S" mint mark (San Francisco):

Very worn - $1,500

Moderately worn - $1,570

Slightly worn - $1,680

Almost no wear - $1,728

Uncirculated - $1,800 to $84,500