The atomic bombs were the product of an industrial effort which cost just under $2bn ($20 bn in 1996 dollars). One billion dollars to destroy a city which would have been destroyed at minimal additional cost by one conventional raid represented an awful lot of ‘bucks per bang.’ Another way to look at it is that it cost $3bn to manufacture the 4,000 or so B-29s which were used exclusively in long-range operations against Japan, including as atomic bombers…Another index was that the total cost of the atomic bombs was the equivalent of making one-third more tanks or five times more heavy guns.
That is intriguing but it misses two points. First, the cost of making subsequent atomic bombs is lower. Second, atomic bombs have superior signaling power about the willingness to destroy. That excerpt is from David Edgerton’s The Shock of the Old: Technology and Global History Since 1900.