What is the value of a 1903 US 20 dollar gold piece?

Your coin is a Coronet Head $20 Double Eagle. In circulated condition its value is determined by the amount of gold it contains. As of 04/2010:

Very worn condition - $1,340

Moderately worn - $1,380

Slightly worn - $1,440

Almost no wear - $1,490

If the coin is uncirculated, look on the back 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 04/2010:

No mint mark (Philadelphia): $1,550 to $20,050 depending on quality

"S" mint mark (San Francisco): $1,550 to $15,190