Inefficient perhaps, but cheap. I've seen some of the slave housing-not much money put into it, trust me - in Virginia, the ones I saw and lived with (one in Stuarts Draft, the other close to Staunton)had packed dirt floors; if all you had to provide was food - and some of the slaves grew their own gardens - and most of the clothing they wore they wove themselves, there wasn't all that much outgo.