We have a rough (if slightly problematic) idea of what is generally used to define poverty for statistical purposes.
Sociologists tend to define the middle class largely in terms of rough lifestyle — access to credit, property ownership, higher education (or the realistic expectation of it), savings accounts, etc.
But, off the top of your head, what would be the criteria — monetarily or otherwise — for you to consider someone rich?
(Shamelessly sampled from Klein.)