The cost of the reactor will be proportional to its size, so the cost/power will be inversely proportional to the power density. Lawrence Lidsky (who was also at MIT) (and also a similar argument from Pfirsch and Schmitter in Germany) famously pointed out back in the 1980s that DT fusion reactors will inherently have terrible power density compared to fission reactors, and this will render them noncompetitive. Despite putative rebuttals at the time, nothing we've seen since contradicts their devastating argument.
Note that Helion wants to go with D-3He, and use direct conversion for at least some of the energy recovery. This might be the only hope for making fusion compete. But of course you need 3He; making it with DD fusion requires even more aggressive plasma physics.
ARC at least isn't quite as absurd as a tokamak the size of ITER, which has a gross fusion power density another order of magnitude lower.
http://orcutt.net/weblog/wp-content/uploads/2015/08/The-Trou...
https://ui.adsabs.harvard.edu/abs/1987oepn.book.....P/abstra...
Note that Helion wants to go with D-3He, and use direct conversion for at least some of the energy recovery. This might be the only hope for making fusion compete. But of course you need 3He; making it with DD fusion requires even more aggressive plasma physics.
ARC at least isn't quite as absurd as a tokamak the size of ITER, which has a gross fusion power density another order of magnitude lower.