Some Americans, such as James Madison, manumitted at their death, despite these costs. Others, like Thomas Jefferson, simply talked about it. The mandatory slave patrols was another subsidy, rather like a tax, of the wealthy slave owners on the less wealthy. These various subsidies make it very difficult to assess the profitability of slavery, certainly in comparison with free labor. The free market is based upon the idea that you are free to offer yourself and your labor on the market.
Slavery is the very antithesis of that idea.
To equate slavery with the market confuses the issue. Thus, massive and growing government intervention was necessary to sustain the slave system. There is a real question, however, as to what extent slavery was a factor in the coming of the War. As a mercantilist follower of Henry Clay, Abraham Lincoln was committed to a massive program of governmental intervention in the economy. From the standpoint of the South, this meant a massive increase in the tariff in support of northern industries.