[jvm]

A lot of the richer cities have major restrictions on development that drive rents very high [1]. The cities that are attracting more low-income people (e.g., the Texas cities) have much more development-friendly policies that keep the rent lower despite massive immigration.

[1] Krugman on the "zoned zone": http://krugman.blogs.nytimes.com/2007/10/20/the-two-americas... (his point about the bubble actually turned out to be probably not completely accurate, but the split in urbanization policy is very real).