Ok I finally got around to reading through this and while I appreciate his attempt to come up with a number, this is pretty lazy. Lots of holes and it's pretty hard to respect his final number.
A) Using population averages is a shortcut. I've read some cliometric work and if I remember correctly the enslaved population spiked during the 1850s and '60s. Using so like saying me, a homeless guy, and Bill Gates have an average wealth of several billion dollars doesn't come close to reflecting the reality, this does the same. This would also account for all slaves working from their day of birth until the day they die.
B) Slaves didn't work all year. Yes they were held in bondage all year but he doesn't account for planting seasons, during which the number of hours worked per day could vary wildly. Slaves were allowed leave to relax and visit family and Sundays were (largely) considered days of rest. (Not often but his model is sloppy)
C) I'm kind of curious as to his interest rate. Just don't know where it came from. Even accounting for the wild swings of business cycles, two percent (absent inflation) is a weird choice. It should probably be higher.
D) My biggest concern: using TODAY'S minimum wage for his calculations. That's not the way it works. With a little digging, you can find average wages for that period. And he doesn't account for skilled labor (yes I know this is meant to be a minimum).
Not mitigating the horrors of slavery or the need for a calculation of the lost wages of the period. I just think this is a pretty lazy attempt at it and it shouldn't be something to go citing as fact.
Cliometrics texts if you're interested:
Time on the Cross - Fogel and Engerman
Slavery and the Numbers: A Critique of Time on the Cross - Gutman
Without Consent or Contract - Fogel