Now say a quantum computer which could implement shor's algorithm on 256bit numbers could be built for $50,000 per TH equivelent. Who cares? An attacker is going to take the more economical option.
The USA government doesn't care of economical issues (it can print a lot of dollars). When existance of Bitcoin becomes a political problem, it will be solved using all resources of USA economy.
In that case, why would they bother to fuck around with unproven quantum technology instead of using their own ASIC?