A nuclear power plant is "just" a steel cylinder in a concrete building, attached to a very ordinary steam turbine hall and a set of transformers.
The expensive part is not the building! That steel is not some unobtanium, and while the concrete is the highest grade, it's not $billions expensive. The fuel pellets are surprisingly cheap also. That's... kind-of the point! You're not burning thousands of tonnes of coal in these things. The low fuel costs are a selling point.
The reason for the expense is because every single nut, screw, and bolt needs to have a mile high pile of paperwork. Because every pipe, pump, and filter needs to be certified. Every sensor needs to be purpose-built and documented by expensive PhD-wielding scientists, not just obtained at the local engineers shop. And so on, and so forth.
The Westinghouse design paperwork specified the type of paint used for the dividing lines between the parking lot divisions.
Not to mention the lawsuits by NIMBYs, which means you often can't even start construction for 6-15 years. No other business in the world is held up in red tape for as long, or as intensely.
PS: This is why China can slap down AP1000 reactors down half a dozen at a time, like they're playing Sim City. Over there, with an authoritarian government, they simply don't have these red-tape / certification problems. Some high-up politicians simply make the decision, and that's... it. Time to start pouring the foundations.
PPS: It seems that on further research, China is having troubles with the high base cost of the plants, even with the reduced overheads due to mandates at the highest levels: https://www.technologyreview.com/2018/12/12/138271/chinas-lo...
... so the truth is probably somewhere in the middle. Nuclear is expensive, but too expensive in western countries. Realistically, they're probably not worthwhile long term unless subsidised by the nuclear weapons industry...