And what sets those upper limits?

In general: money. Buying more of the most performant equipment available.

So. It's an economic designation.

Sure, you're limited by money. But if someone magically was able to produce a machine for $100 that was on par with Titan (http://en.wikipedia.org/wiki/Titan_(supercomputer)) I would still call it a "supercomputer" until it became ubiquitous.

Except that someone would buy $10 million of such machines and gang them together. And we would call that a supercomputer, not whatever you put under your desk.

Defining it on any architecture or performance metric is just pointless because the march of time renders such things utterly moot. Remember when PlayStation 2s were "supercomputers"? Please.

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.

What doesn't slide is that supercomputers cost more.

Here's a simple test.

Suppose that I give you these two measurements:

SPECmark: 1.2 million.

SPECmark: 3 million.

Which one is the supercomputer? Without knowing the date, there's simply no way for you to tell.

Suppose instead I write:

System cost: $20 million 1990 dollars

System cost: $400 1990 dollars

Which one is the supercomputer? I think most people will be able to pick which one is which.

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.

If they only had one, it doesn't matter how much it cost them to make, it's worth much much more than $100 dollars to someone. If there were enough supply that it were possible for it to be only $100 then it would have to already be ubiquitous. I agree with jacques_chester that it's an economic distinction.

Change "able to produce" with "willing to sell for" and my point remains the same.

I agree that you're not going to get a "supercomputer" for less than about $100,000. But supercomputers are defined by what they can do. Their cost is secondary. Necessary in a world without magic, but secondary. I can spend $100,000 on a computer, but that alone does not make it a "supercomputer".