“I doubt that it will cost any more than fuel elements for, say, a fission reactor.”
Maybe, but the fission fuel produces a lot of power even though the fuel is expensive. The fuel for this type of reaction could be much cheaper, but if the power produced is much less, it could still end up being less efficient.
Yes, but if it becomes cheap "enough", lower efficiency won't be a problem. After all, we humans used grossly inefficient steam engines for a LONG time and accomplished quite a bit with them (an our automobiles are "still" not all THAT efficient). But I expect both the amount of needed "active agent" and the cost to drop precipitously with time.