Why are nuclear weapons still so expensive?

There are no nuclear weapons of any type, regardless of explosive yield, as far as I can determine, that cost less than at least a few million dollars. On the other hand, I believe no nuclear weapons actually cost more than a few tens of millions of dollars, again, regardless of explosive yield. And, explosive yield on nuclear weapons varies from just a few tons of TNT, to tens of megatons of TNT. Why is this exactly? Obviously, this consistency of cost, regardless, of explosive yield, doesn't apply to conventional explosives, at all -- an almost identical cost, over six orders of magnitude of explosive yield. And, despite the fact that nuclear weapons have existed for almost eighty years now, the costs do not appear to have gone down at all, which is rather odd for a very old technology. So, is this cost factor in nuclear weapons simply an inevitable technical obstacle, that cannot be gotten around, somehow? Or, is this a socio-economic effect of some sort? After all, governments are very frightened of nuclear weapons, indeed. That's why there are such tight controls over the possession of even raw Uranium ore -- in the U.S., no one is allowed to possess more than a few pounds of raw Uranium ore. So, there are no private entrepreneurs working on nuclear energy, at all. I'm thinking particularly in terms of potential practical applications of nuclear explosives, such as Project Orion, for space travel: https://en.wikipedia.org/wiki/Project_Orion_(nuclear_propulsion) It would certainly be quite useful for such projects if A-bombs could be produced for only ten thousand dollars, or so. On the other hand, if anyone with ten thousand dollars could purchase an A-bomb, that might make government officials worldwide very nervous, indeed. So, are governments quite deliberately making it very difficult for anyone to develop cheaper nuclear weapons, quite intentionally?