Fission is not cheap if you build a new nuclear plant, not in the Western world. That's why almost no nuclear plants are being build. Making a safe plant is just really, really complicated.

Second, this specific argument can be used to see why fusion is a pipe dream. The primary competitor to fusion is fission. And the fuel costs of fission are pretty low, as you just said. So fusion will not be competitive unless you can built them around the same price as fission plants.

Someone else in this thread talked about S curves. Well, those kind of S curves happen for tech that gets produced in larger quantities, where it is economical to spend engineering resources making the production of the tech cheaper.