It really depends on the area. I would classify my current salary as low-end for middle class. For your (low-end) basics (travel, food, accommodation, health insurance) it's around $24K a year in the slightly more expensive areas. Gasoline costs typically offset accommodation costs - especially now that it's around $45 to fill a 35L/9.2 gallon car tank.

South Africa is pretty strange in the sense that certain areas have a huge effect on living costs - you might pay 10%+ less on groceries simply by driving to another suburb. Some cities (ex. Pretoria) are more prone to be targeted by the government for money making purposes - ex. increasing taxes in Pretoria to pay for new roads in Durban (600Km/~400 miles away).