The conclusion that DT fusion reactors will be expensive follows from basic engineering considerations on heat transfer, not from specifics of any design. The volumetric power density will be lousy, so cost will be high.
> A modern nuclear plant costs about $2/Wth
This may have been the projection before the AP1000 and the EPR flamed out. In practice, the cost can escalate well above that. Their complexity made them difficult to build. And yet, these designs have reactor power densities orders of magnitude better than ITER, and are far simpler (and more fault tolerant) than a fusion reactor would be.
The cost is not a function of volumetric power density. I believe we have had this conversation before.
The most expensive nuclear plant projected to be constructed is the Hinkley Point C reactor. With all its overruns, it's projected that the cost will be £23 Billion, which works out to $3.37/Wth. $2/Wth is a $6 billion 1000 MWe plant, quite typical. Note that this is for reactors in the west, in China, where non-technical issues like NIMBYism aren't a concern, the cost is closer to $1/Wth.
> The cost is not a function of volumetric power density. I believe we have had this conversation before.
Yes, and you failed to make the case that larger things aren't more expensive than smaller things (all else being equal, but note that fission reactors are made of steel, not the complex sophisticated equipment of fusion reactors). We can continue the argument here, if you can come up with any argument for your position that makes any sense.
> A modern nuclear plant costs about $2/Wth
This may have been the projection before the AP1000 and the EPR flamed out. In practice, the cost can escalate well above that. Their complexity made them difficult to build. And yet, these designs have reactor power densities orders of magnitude better than ITER, and are far simpler (and more fault tolerant) than a fusion reactor would be.