The Manhattan Project wasn't even the most expensive US development project of the war - the B29 cost about $2 billion.
What comes to mind is that it’s pretty important to keep an airplane in the air, which must require a lot of careful engineering. Also that you have to make a lot of them. A bomb doesn’t really have either engineering requirements. (Only has to work once, they only made a few)
I also wonder to what degree cost was saved by getting all those highly talented and motivated refugees from europe
Prior to that bomber crews operated using electrically heated flight suites, gloves and used oxygen masks, when reaching those altitudes.
http://www.303rdbg.com/uniforms-gear3.html https://en.wikipedia.org/wiki/Boeing_B-29_Superfortress#Pres...
After Fat Man & Little Boy, it's obvious we didn't just stop building bombs. The Manhattan Project's main development was the proof that the bomb was possible. Much more money followed making new versions that were even more powerful.