http://news.yahoo.com/s/ap/20100610/ap_on_re_us/us_slavery_photo A little boy named John was sold for $1,150 in 1854. That was about 57.5 oz of gold(gold was $20 per oz then). In todays dollars thats about $70,000. A plantation owner could hire someone to work 12 hours per day 6 days a week for 5 years with that much money. Thats not even including feeding and housing a slave. Add that in and you are probably looking at 8 years just to break even. If a slave gets sick & dies, or runs away before that first 8 years, they lost money. After the 8th year I guess they did ok, but seeing how the average slave died after 20 years of service, it doesnt seem worth the risk to buy slaves to save 60% on labor costs over that time frame.
Throughout history, economic powers have been built on the back of cheap labor... slavery being but one form.