Of course, this also depends on what part of the world, and, if you're talking the United States, what part of the country. Living standards vary widely between New England, the West Coast, the Midwest, and the South (not to mention Alaska and Hawaii).
I would state that a healthy middle class, with the wife and 2.3 children, makes between $50,000 and $80,000. Lower middle class is $30,000 to $50,000 and upper middle class is $80,000 to $200,000. Anything above that and you really are out of reach of most of this nation's income earners.
All of these statistics come from one wage earner (e.g., one job).
Melon