[jjk166]

It's not unreasonable to compare them, but that comparrison must be made in context: we're saying that something that makes up a very tiny part of the cost will be more expensive, while something that makes up a huge part of the cost will be dramatically less expensive.

Instead of a $100 million reactor, you're looking at $1 billion in reactor spending, but instead of a $4 billion dollar plant that this reactor goes into, you're looking at a $2 Billion plant.

A fusion plant would be comparable to a very expensive fission reactor in a world where people weren't afraid of fission plants, but in that world a fission plant would be dirt cheap. In the real world, fission is way more expensive than the engineering challenges would imply. It's not the materials or the containment that is expensive, it's having all of your assets sit idle for years on end while yet another environmental impact study is conducted.

Also, some of your assumptions are unreasonable. For example the reason you need a containment building around a nuclear reactor is that you can't just vent to atmosphere, because the water contains large amounts of tritium. It's perfectly fine to just vent helium to atmosphere in case of an emergency as it's not radioactive. While a fusion reactor would use a lot of tritium over time, at any given moment the amount present is rather miniscule, it is being actively generated on site specifically and if anything the major technical issue is not having enough. A reactor the size of ITER would have approximately 0.6 g of Tritium in the reactor at any given time, losing all of that to atmosphere would be equivalent to approximately 2% of the annual tritium release from The Hague Nuclear Reprocessing plant. Decomposition of titanium hydride at the temperature of the molten salt is slow, while obviously undesirable, there is no danger in the magnets decomposing, the real issue is quenching, which is one of the few genuine safety concerns of a fusion reactor.