Why larger naval guns came with higher shipbuilding costs

Why did construction costs rise so much between battleships with 13.5-inch and 15-inch guns?

Larger guns came alongside changes in speed, armour, and other parts of the design, so calibre alone cannot explain the price increase.

The price increase between these battleship designs covered more than their main armament. Speed also increased, and larger guns commonly arrived with greater protection and other design changes. Each added to the total construction cost. Comparing gun calibre alone leaves out part of the explanation.