The South’s enormous economic stake in slavery far outweighed the impact of protective tariffs on its income. In 1860, the aggregate value of slaves as property was $3 billion, nearly 20 percent of the nation’s wealth. The value of slaves was more than 50 percent greater than the capital invested in railroads and manufacturing combined, a calculation that excludes the value of land in southern plantations. Slavery generated a stream of income that enable overall white per capita income in the south to approximate that of northern whites. In the seven cotton states, nearly a third of white income came from slave labor.
That is from the new Douglas A. Irwin book on trade policy, Clashing Over Commerce: A History of US Trade Policy.