Well, obviously it's not the most representative example, but yes, if a country intends to kill hundreds of thousands of people, then an atomic bomb is probably the most cost-effective way, even after accounting for R&D. Moreover, if the calculus is how to win the war with the lowest number of additional lives lost, the atomic bombs dropped on Japan were quite likely significantly less deadly, even when comparing just against the expected number of Japanese civilian casualties from the alternative scenario of a Normandy-like invasion of Japan.
EDIT: It's worth saying that humans have been killing each other from the dawn of humanity. Studies on both present-day and historical tribal societies generally show a significantly higher homicide rate than what we're used to seeing in even our most dangerous cities and across our biggest wars.
> if the calculus is how to win the war with the lowest number of additional lives lost, the atomic bombs dropped on Japan were quite likely significantly less deadly
This is just US propaganda. These numbers come from the fact that the US was "anticipating" a ground invasion of Japan or vice versa.
Which, to be clear, was always a made-up alternative. By the time the atomic bomb was dropped, Japan had already tried to surrender multiple times, both to us and the soviets. The reality is we just wanted to drop an atomic bomb.
You are probably right that Japan were close to surrendering and had begun some signaling around it, particularly via the soviets, but my understanding is that they hadn't actually done that, and according to the sources I read, they absolutely weren't willing to unconditionally surrender and demilitarize before the bomb.
But I don't understand why you put the ground invasion plans in quotes - are you claiming that all the effort spent on Operation Downfall[0] was just a misdirection intended to fool everyone, including the high-ranking officers involved in the planning?
I don't think it was a misdirection, but I do think it was an obviously bad idea that should've never materialized, and it didn't. I'm arguing it wouldn't have materialized anyway, and if it did, it wouldn't have been necessary.
Things did work out for Japan in the long run, but I still believe a conditional surrender + no atomic bombs should have been the solution. The US was very greedy with its demands, and I think a large part of that is our history of militarism and our desire to use new weaponry. The atomic bomb was already made, and I think realistically we were just itching to use it.
ps- include the technology built to kill the enemy-labor in large numbers. Start with the Atomic Bomb in Japan.. that saved a lot of labor, right?