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.

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.