[pas]

The cost of development is amortized over infinity. (Sure, on the other hand the NPV of a perpetuity bond is finite.)

Of course (economically) the optimal allocation of current resources/funds probably would not involve fusion research, but instead instant (ASAP) replacement of coal, oil, gas power plants. Then working on fusion. But if we look at what fusion researchers should do, either they could be retrained or we can simply go ahead and print some money to pay them to work on what they want to do anyway (fusion research).