In a recent post it was said that it involved huge sums and people, and that Germany didn't think it could be done while at war.
Here's a breakdown of the Manhattan Project costs:
You can see from that that 80% of the expense went to the producing of fissile materials (the fuel for the bombs), and of that expense, about 75% went towards constructing and building (as opposed to operating) the plants.
And here is a breakdown of how many people worked on the project:
Here you can see that those same facilities employed the bulk of the people involved (some 540,000 people), and that most of those employees were involved in construction.
Why were construction costs so high? The facilities were massive. Here are pictures and schematics of the K-25 facility — just one of the Oak Ridge facilities that was used to enrich uranium (they pursued three separate methods for enriching uranium at Oak Ridge, and built one reactor). At Hanford, they built three industrial-sized nuclear reactors from scratch, plus several "canyons" for chemically stripping out plutonium from spent, radioactive reactor fuel, plus lots of other more minor facilities.
At a technical level, enriching uranium is hard — it still is, even today. It was way harder in the mid-1940s when nobody really knew which method would work. And while running industrial-sized nuclear reactors isn't that hard today, it certainly was when none of them had ever been built before. They jumped immediately from building a single small-scale reactor to building three gigantic ones. It was a non-trivial undertaking (and did have numerous technical problems associated with scaling it up).
And for all of those construction laborers, and plant operators, they had to provide housing, infrastructure, amenities, and so on. Why? Because for security purposes, the sites were isolated, and they wanted the workers to be relatively contained, also for security purposes.
Enriching uranium at Oak Ridge Tennessee and setting up a Plutonium production facility at Hannaford Washington required the construction of two massive industrial facilities. A more detailed look at the construction of the major Uranium enrichment and plutonium production facilities can be found in "A Call to Arms: Mobilizing America for World War II" by Maury Klein. Essentially, two small cities had to be built from scratch, during a period of wartime material shortages.