Actually, 52 years since he is including 20 years for his wife as well. Comes out to $3846/person/yr in today's dollars if the above calculation is correct, which is considerably lower than what I'd sell myself into slavery for.
Of course, this doesn't take into account things like income disparity between countries and so on - that is, median income in Bangladesh is very different than the US today, but this was Antebellum south, at a time when America was still a developing country.
All this to say - maybe $4k/year was a "fair price."