Tokamak style fusion is efficient only at extreme scale. That's not five 10GW nuclear power plants, it's one terawatt scale plant. For reference, the entire Three Gorges Dam and its electrical plant cost ~$30bln and it has a nameplate capacity of 22.5GW. $50B seems low for a terawatt-scale fusion plant.

You'd need 3 of those plants to replace all electrical generation on earth, assuming they're at a 95% capacity factor. So call it a trillion dollars, maybe. Then you've got to look at total energy usage, which is order of magnitude larger, at 120TWh, so you'd need 12 to supply every energy need of current civilization. Maybe ~$50 trillion all included.

So one year of the entire world's productive output.

that would be shitty US prices cause we rarely reuse the same design.