Marshall County has the lowest cost of modest living for a family of four in the U.S., according to a new study by the Economic Policy Institute. The average monthly rent in the small city of 7,900 people is about $500 a month, and spacious houses can be snapped up for less than $100,000.s for housing, families in places like New York and San Francisco fork over as much as 25% of the budget for shelter — while the eight lowest-cost areas for a family of four are all counties in Mississippi.