With the cost of buying houses and apartments rising, more people are renting. Some 36.1 percent of households are renting today, compared to 34.5 percent in 1980. Runzheimer International reports that rental costs are highest in the greater Boston area. The annual rental for a six-room, three-bedroom, two-bath, 1,500 sq. ft. townhouse here averages $16,277. The same unit would cost $14,767 in San Diego, $14,717 in Miami, $13,629 in Minneapolis, $11,364 in Phoenix, $10,743 in St. Louis, $9,961 in Houston, $8,079 in Pensacola, and $7,497 in Oklahoma City.