How much is the $20 Saint-Gaudens Golden Double Eagle worth?

The Saint-Gaudens Double Eagle is worth at least $1290 in pure gold alone, (at 0.96750 troy oz gold), so that is the lower limit. Uncirculated 1920(S) coins, 1921 coins, and 1927(D) coins are worth $25,000, $50,000, and $500,000 USD respectively.