Tariffs were levied on imports, with higher taxes on luxury goods.
The north paid more tariff, as shown by payments collected in northern ports vs. southern ports.
The south had more rich people, who demanded a lot of luxury goods.
http://www.amazon.com/For-Good-Evil-Impact-Civilization/dp/1568332351
Charles W. Adams, a U.S. tax attorney
“...the South paid about three-quarters of all federal taxes, most of which were spent in the North.” [p. 326-7]