I'm not sure about those direct costs. According to my references, a male slave in 10th-century Scandinavia cost about as much as a horse, a female slave about two-thirds as much; that's a pretty good chunk of change but it doesn't seem obviously out of line with the value of labor after externalities. I don't have figures offhand for any other slaveholding cultures, but the impression I get is that the pure exercise of power was not the main determinant of value in most, if not all, of them,