What is the value of an 1878 US 20 dollar gold piece?

According to USA Coin Book, the type of coin minted in 1878 is the Coronet Head $20 Gold Double Eagle. It was minted in Philadelphia, Carson City and San Francisco.

Philadelphia (no mint mark): In very fine condition, this coin is worth about $1,340 up to about $1,675 in MS60 mint condition. In MS63 choice uncirculated grade, this coin is worth about $11,060. Proof coins were also made here and are worth about $37,800 in PR63 choice proof grade.

Carson City (small "CC" mint mark): In very fine condition, this coin is worth about $2,600 up to about $31,000 in MS60 mint condition. Not many coins are known to be in better condition but would be worth an extremely large amount of money!

San Francisco (small "S" mint mark): In very fine condition, this coin is worth about $1,340 up to about $1,840 in MS60 mint condition. In MS63 choice uncirculated grade, this coin is worth about $18,630.

The gold content value of this coin is currently about $1,300 as of 10/2010.