[jimbobthrowawy]

I think the Manhattan project had a lot of consideration towards making it economical. (and huge government programs weren't quite so common back then) Plus, I think things that were returned/recycled weren't counted against the total. e.g. the $600 million worth of silver they borrowed to make calutrons, because there was a copper shortage.

[egl2021]

Economy was not really a consideration. The project developed two different bomb designs (plutonium and uranium) in parallel. They tried three different paths to fissile material: gaseous diffusion and electromagnetic separation for uranium and reactors for plutonium.