What dollar value is the gold doubloon nailed to the Pequod's mast?

The gold doubloon nailed to the Pequod’s mast is worth 16 dollars.

Captain Ahab offers the coin as a reward to the first crewman who sights Moby Dick. In Chapter 36, “The Quarter-Deck,” he holds it up and identifies it as a Spanish ounce of gold, explicitly calling it a “sixteen dollar piece.” The doubloon becomes a visible symbol of the hunt and of Ahab’s effort to bind the crew to his private vendetta.

The coin receives an entire later chapter, Chapter 99, “The Doubloon.” Several members of the crew study its engraved images and interpret them according to their own personalities and beliefs. Ahab, Starbuck, Stubb, Flask, and Pip each see something different in the same object, reinforcing Melville’s interest in subjective meaning.

A doubloon was a large Spanish or Spanish-American gold coin, and the novel’s coin is often identified with an Ecuadorian eight-escudo piece. The reward is not 5, 10, or 20 dollars: the text itself gives the amount as sixteen dollars, though its historical purchasing power was very different from the modern equivalent.