Slaves probably averaged, in 1860, about $750. Total value of all slaves was around $3B. Which was about 48% of all wealth in the South, and around 19% of all wealth in the country.
I am not going to contradict it because I have no better idea. Now is that $750.00 in 1860 dollars, or $750.00 in 2015 dollars?
If it was 3 billion total valuation, that would put it at about 4 million slaves, which I think isn't far off.