There is no use for the gold bullion, other than to sell it for $150.
Note: prices listed on this page are based upon the player having neutral honor and playing the game on Normal difficulty. If the player has high honor, the selling price will be Increased by 50% everywhere except Thieves' Landing. Low Honor decreases selling price by 50% everywhere except Thieves' Landing. While playing the game on Hardcore difficulty, money is more scarce and the base, neutral honor price of each item will be slightly lower.
This may also be a reference to Senator and President John C. Calhoun from South Carolina. Calhoun was an ardent supporter of States' Rights and the right of a State to secede if its people believed the other States were not fulfilling their Constitutional obligations.
This may be the treasure that Seth was looking for in his third mission. Instead, he found a chest with a glass eye in an upstairs room.