We know the cost of living varies considerably across the country. It can even vary significantly just across town. But what really drives these differences? In a word, housing. Sure, the price of going out to eat can vary by a few bucks per plate here and there, plus we know haircuts and accounting services differ as well. But as seen in this edition of the Graph of the Week, prices across the country for most goods and services fall within a range that varies by 20 percent or so.