The Soviet and other later nuclear weapons programs were significantly less expensive/complicated to reach equivalent capabilities, because they didn't pursue several paths simultaneously like the Manhattan Project did. They also weren't so rushed.

For example, the USSR built plutonium production facilities and tested a working plutonium-based implosion bomb before they produced highly enriched uranium. The Soviet uranium enrichment program was also simplified compared to the US: they built out the most effective technology that the American program demonstrated (gaseous diffusion). They developed the marginally effective calutron enrichment process only to a trial scale and ignored the practically useless liquid thermal diffusion enrichment process.

China choose two paths as well[0], and while at it building the largest man made tunnel complex in the world[1].

[0] https://nsri.nebraska.edu/-/media/projects/nsri/docs/academi... [1] https://en.m.wikipedia.org/wiki/816_Nuclear_Military_Plant

True, but I would note the Soviets chose to do things like put nuclear reactors and Pu production complexes underground to protect them from American air strikes. The sheer scale of their efforts and the added requirements for things like undergrounding entire plants certainly pushed their spending sky-high.

They also had the benefit of having stolen information on what did and didn't work, via spies like the Rosenbergs. Why try what doesn't work if someone else has proved the path before

> They also weren't so rushed.

The threat to the US was that, unless they are first to develop nuclear weapons, they’d risk the war could end up being fought on their land.

I've always assumed that gaseous diffusion required a process similar to silicon etching in order to achieve the pinhole sizes required.