From RealTrends.com: RealtyTrac recently analyzed the affordability of renting and buying in 1,586 U.S. counties and found a surprising number of Americans living in markets that are unaffordable by traditional standards.The good news is that buying and renting are affordable for median income earners in the majority of counties across the country. We considered a market affordable if the monthly cost of rent or mortgage payment represented less than 30 percent of monthly household income for median income earners.
Fair market rents (as provided by HUD) for three-bedroom homes are below that 30 percent threshold in 1,264 of the 1,586 counties, while mortgage payments for a median priced residential property — including taxes and insurance — are below that 30 percent threshold in 1,508 of the 1,586 counties analyzed.
Read more: 47 Least Affordable U.S. Housing Markets