How much did using the atomic bomb on japan cost for the US in World War 2?

It has been argued that the atomic bomb cost aound 2 billion dollars for the 3 bombs: 1 test bomb and the 2 actaully dropped

Almost every penny of that was spent on the industrial infrastructure needed to make the bombs. The cost of the bombs themselves was virtually negligible compared to that. 24 bombs were scheduled for production in 1945: 1 test bomb and 23 to be dropped on Japan. It is unlikely these extra bombs had they been produced would have raised the cost much at all.

The actual cost of a nuclear weapon's physics package is still classified.