First, the power output of a fusion reactor is limited by what the first wall can withstand. Therefore, the power goes as radius^2, where as the cost goes (at least) as radius^3.
Second, the larger a fusion reactor becomes, the more parts it has, and the more reliable each individual part has to be (since there will be no redundancy in many; an leak into the vacuum chamber shuts down the reactor, for example). Reliability is expensive, so each part becomes more expensive as it is required to be more reliable.
Renewables sources are extremely redundant, so they'd scale just fine even with unexceptional per-unit reliability.
Most of your cost difference is that a commercial building isn't just taller than a house, it's also wider. Instead of taking up a third of the lot, it uses the whole thing, and then has 30 times more interior space despite being only 10 times taller. They're also built to commercial building standards which are more expensive to meet.
The costs start getting non-linear when you get into extremely tall buildings that pose special engineering challenges, but nobody is talking about building a fusion reactor into a skyscraper.
> I have no idea why you even imagine that it's possible at all to construct a nuclear power plant out in the ocean. There is certainly no precedent for anything even close to that.
Nuclear submarines survive the ocean just fine.
Yes, they are and it is a huge political issue in nearby countries.
https://en.wikipedia.org/wiki/Russian_floating_nuclear_power...
That's not how construction works. Past some small scale, construction cost scales quadratically or worse with size. Building a 100m tall sky scraper is not 10 times as expensive as building a 10m tall 3-story house - it is at least a hundred times more. The only reason why it's sometimes worth it is in ultra-high land-cost areas, such as Manhattan. But you'll never see sky scrapers outside city centers, because construction costs scale horribly with size, even for simple structures. Building a fusion chamber twice the size of ITER would likely be a new 50 to 100 year research project.
> You keep saying "quadrupling your construction costs" without evidence. We build oil platforms in the ocean on a regular basis. They cost some tens of millions of dollars. Existing fission reactors cost some billions of dollars. The difference from being on the ocean is evidently not the dominant cost. And then you don't have to pay for land.
Is anyone building fission reactors, or any kind of huge concrete building out in the ocean at all? Extracting oil from the ocean is many times more expensive than extracting oil on land. I have no idea why you even imagine that it's possible at all to construct a nuclear power plant out in the ocean. There is certainly no precedent for anything even close to that.