>> what income level can be justifiably considered rich?
One way to deal with this issue more objectively is to define the
"rich" as "independently wealthy," which I would define as owning
enough net worth to earn an average income in a given society.
Let's do some seat-of-the-pants (not enough time to get it right)
estimates: the minimum wealth needed to be defined as wealthy = (the
average income)/(interest rate).
If we use the median per-person income for the US in 2008, this
indicates that the poorest rich person owned about $691,000 in net
worth; if we use the (mean) per capita real income during the same
period, a rich person would have to have owned about $911,000 in net
worth. For a family of four, multiply these by four, etc. Someone with
better access to data can calculate what percentage of the US
population would count as rich from these.
I used the interest rate on tax-free municipal bonds (4.8%, to correct
for the role of taxes) and ignored the role of inflation (which was
very low in 2008).