I would say state median income, muon. Wealth in MS compared to MA is definitely a different standard, so national doesn't always work. However, it would be unfair to call someone who lives in a super wealthy county or suburb poor when they can simply move to another nearby town. It's much easier to move to a different locale within a state than move to a different state.
I'm not sure a state median works so well in IL. Jackson county, home to Southern IL University, has a median household income well below that of MS. Most of southern IL is around that same level and central IL is generally below the national median. However Cook is just below the state median and Chicagoland overall is above it and well above the national average.
The upper 5% of household incomes has less variation as a fraction of the US value than the median and might be a better measure of "rich". Using the upper 5% also puts Cook on a par with the suburban counties (209K in Cook and 203K in Kendall) and the US as a whole (191K), yet still differentiates that area from the rural counties in IL (131K in Jackson, IL). 200K would not be a bad approximation if one needed a single number.