Economic Reflections on Slavery
Only an economic idiot would think that the US was made rich by slavery. Slavery is less efficient than free labor---and slaves were not cheap. Frankly, giving the slave trader title to a person's labor doesn't do anything but transfer funds---no net gain could ever result: In fact, the reduced incentives make it more expensive. The only advantage that owning a slave had over procurring labor freely was the slave owner's claim to the progeny of his slaves: But this value would likely already be built into the slave's market price---so it was little more than an investment vehicle, which being non-capital in nature was less useful than other modes of investment.
Of course, investment took this form largely because Northern impositions on the South---esp. tariffs and higher railway freight rates that followed directly from government regulation---acted to keep capital out of the South. I am not arguing that slavery is good---far from it. In fact, I am arguing that no good in aggregate---even if we consider only the effect on whites---accrued to anyone other than slave traders and to a lesser extent slave owners. Slavery was a net drain---this is, after all, why the South lost despite being on the defensive (which generally confers an advantage) and having better officers. If slavery had created riches, why didn't those riches win the war for the South?