What is the value of an 1885 US 20 dollar gold piece?

Look on the back of the coin to see if there's a small mint mark letter below the eagle. It may be blank or there may be a CC or S.

Numismedia lists the following approximate retail values as of 01/2012:

No mint mark (Philadelphia):

Moderately worn - $5,820

Slightly worn - $8,440

Almost no wear - $36,178

Uncirculated - $61,750 to $141,750 depending on quality

>> If your coin doesn't have a mint mark it should be inspected by an expert to make sure it isn't a fake.

"CC" mint mark (Carson City):

Very worn - $2,160

Moderately worn - $2,500

Slightly worn - $3,440

Almost no wear - $10,292

Uncirculated - $19,830 to $36,730

"S" mint mark (San Francisco):

Very worn - $1,870

Moderately worn - $1,900

Slightly worn - $1,910

Almost no wear - $1,946

Uncirculated - $2,100 to $28,690

DISCLAIMER: The retail values quoted are the best available as of the date shown, but may vary significantly due to changes in the precious metals market. Also the wholesale (buying) price of a coin will be less than the selling (retail) price. A reputable coin dealer will be able to give a more accurate valuation based on inspection of the coin.