For an interesting look at housing and transportation costs around the United States, check out the Center for Neighborhood Technology's Housing + Transportation Affordability Index. After selecting a region, it becomes quickly obvious that city dwellers spend less overall (as a percentage of their total income) on housing and transportation than do suburban inhabitants. I imagine this is most likely due to the increased availability of mass transit in urban centers, which allows people to live without cars (thereby significantly reducing their transportation costs). Interesting stuff ...