What a fourth battleship turret really costs

How much more would a conventional twelve-gun 16-inch battleship cost than a nine-gun design?

The premium centers on the extra turret and the hull and personnel needed around it; propulsion need not grow proportionally if the longer hull is efficient.

The extra turret adds weapons, ammunition, protected volume, and crew-support costs, but a longer hull does not automatically require proportionally more machinery. Beam and hull efficiency can matter more to power demand than length alone.