Expensive

I'm surprised by the number of people in comments below who have signalled that they think it's ludicrous that a family of four can find it hard to live on $90,000. There are several housing markets where they would certainly find it difficult to buy anything resembling a 4 bedroom house - San Francisco area, Orange County, San Diego, increasingly the DC area, Boston, etc... In my post below I meant to write:

There are still many parts of Philly metro area in which "upper middle class" is certainly achievable for $90,000 in household income. Not true for SoCal, or NY, or Boston, or SF,a stretch for Chicago, etc. In much of America 90,000 is more than enough, of course.

which explains some of the response. And, I'm not saying that they couldn't afford to rent something that their family could survive quite comfortably in. But, the original point was that this family certainly wouldn't be "well off" or "rich" or even "upper middle class" by the standards we usually set for these things in terms of media images. We're at the top of a housing bubble (it may not pop, but I can't see it continuing to rise), so it's generally cheaper to rent in these areas than to buy. But, a household income of $90,000 in Orange County, CA at absolute best affords you a "middle middle class" lifestyle, due to the cost of housing (unless, of course, you bought into the market decades ago. )