Well, let's be clear: a mainframe, a cluster and a supercomputer are all different things.
But they do have certain characeristics in common. All are power-hungry and eat real estate. And, unless you go for obsolete kit on its way to the scrap heap, they are all stupidly expensive and way beyond the needs of the simple home user.
And fun if you like that sort of thing. I'd love one of those old Crays that were built inside a circular sofa.
"Klinger, do you know how many zoots were killed to make that one suit?" — BJ Hunnicutt, 4077 M*A*S*H