Detroit, with a median rent to price ratio of 1.22, is correct if we only consider markets over 100k in total population. At 25k and over in pop, the highest cash flow market would be Flint, Michigan with a rent to price of 1.81 If we don't filter for population at all, the answer is actually Benns Church, Virginia. The reason for this is this census designated place is almost fully comprised of mobile homes worth a median of 11k with contract rents being reported as $950.
