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.
Here’s a cut and paste from MeasuringWorth.com.
None of these prices has much meaning to us today, but they would if we revalue them in today’s dollars to the amount of money slave owners spent 150 years ago. The techniques developed in MeasuringWorth have created ten “measures” to use to compare a monetary value in one period to one in another, as explained in the essay “Measures of Worth.”. Of those ten, three are useful for discussing the value of a slave. They are: labor or income value, economic status and real price. Using these measures, the value in 2011 of $400 in 1850 (the average price of a slave that year) ranges from $12,000 to $176,000.