From a 3-year old article on Thomas Jefferson in The Smithsonian Magazine:
A startling statistic emerged in the 1970s, when economists taking a hardheaded look at slavery found that on the eve of the Civil War, enslaved black people, in the aggregate, formed the second most valuable capital asset in the United States. David Brion Davis sums up their findings: “In 1860, the value of Southern slaves was about three times the amount invested in manufacturing or railroads nationwide.” The only asset more valuable than the black people was the land itself. The formula Jefferson had stumbled upon became the engine not only of Monticello but of the entire slaveholding South and the Northern industries, shippers, banks, insurers and investors who weighed risk against returns and bet on slavery. The words Jefferson used—“their increase”—became magic words.