What is the value of a 1900 US 20 dollar gold piece?

This coin's value will fluctuate along with the price of gold. 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 an S.

Numismedia lists the following approximate retail values as of 03/2011:

Circulated coins, regardless of mint mark:

Very worn condition - $1,670

Moderately worn - $1,700

Slightly worn - $1,730

Almost no wear - $1,766

Uncirculated, no mint mark (Philadelphia)

Uncirculated - $1,840 to $23,020 depending on quality

Uncirculated, "S" mint mark (San Francisco):

Uncirculated - $1,840 to $43,880

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.