I don't really think that per-capita income is a good measure of how wealthy a given area is; I think on per-capita income, the wealthiest county in NY is New York (Manhtten) ? Which is an absurdity when you consider quite how poor large tracts of the borough are (in both relative and *absolute* terms).
IMO a wealthy area shouldn't include large numbers of poor (or even working class!) people.
The percentage of poor in Manhattan is much lower than in rural areas, or even in Brooklyn, even if you adjust for cost of living.
The thing about wealthy areas like what you looking for - consistently wealthy - is they're too small to cover a whole county. That's like saying Surrey is the wealthiest county in England, but I'm sure it contains many workers as well.
