Talk to Me Like I'm Stupid: A Census of American Slavery

I received an exceedingly kind e-mail, which included the following:

The quotes from the time concerning why the Southern states chose the path they did is always important in the face of the "state's rights" historical revisionism. And your comment concerning African-American's 250 years under existential violence brought to mind something I had not considered before. 

 Are there any figures on the total number of men, woman and children who lived and died under slavery during those 250 years? It would seem to me that the figure must approach if not surpass the 600,000 figure of those who died in the Civil War. I cannot escape the feeling that the cost in lives to end slavery was anything other than a debt repaid.

I want to move away from a comparison, and dig right into the question posed here. We have plenty of doctoral students, and historians who post on this blog. I'm wondering if anyone has done any calculations on the number of black people who lived in slavery during it's 250 year existence in America (or what would become the United States.) This would obviously predate the United States, and it would likely include areas that weren't even English--Louisiana, New York and Florida come to mind.

Have any of our historians have data on this? Are there just too many variables? Any thoughts would help.