The Economist surveyed the literature on the economics of slavery:

• Businesses that used slaves were profitable. Returns on investing in slaves were comparable to those from investing in other capital projects such as railroad bonds.
• Slavery also drove the rise of provincial banking due to the need for credit in the international slave trade.
• Profits from slavery were used for important infrastructure projects such as railways.
• However slavery was likely a net negative for regions as a whole.
• Slaves – understandably – made reluctant workers and thus depleted farming soils more quickly.
• They were also unable to develop expertise and drive innovation as they weren’t given the opportunity to engage in scientific enquiry. As a result of this southern farms were ultimately eclipsed by northern ones in the United States.
• Since slaves were a fixed cost (in terms of food and shelter) farms couldn’t be dependent on any one crop as a crash in its price may make them unable to pay their fixed costs. Therefore farms were forced to diversify and this inhibited specialization and trade.
• The lack of specialization ultimately meant that the American South had troubles developing a manufacturing industry.
• And, of course, when considering the economics of slavery it is important to consider the moral and ethical costs, which, of course, eclipse all other costs and benefits.

Read more about why slave owners might not have been profit seekers, other industries that blossomed as a result of slavery, and more over here.

Source: The Economist