Note that I said it was a sliding performance target. That is, I agree that defining it as any particular architecture or absolute performance is pointless. It's a designation relative to what is currently possible.
Relative performance is how people who design and use supercomputers define them. Cost is a secondary effect because if you're going to shoot for the limits of what we can do, it's going to be expensive.
The major difficulty with your second list is that expensive computers don't need to be high performance. Consider the computers that go into satellites and spacecraft. They are extremely expensive, but not high performing.
