"What was the deadweight loss of slavery? Using data on estimates of earnings of free workers, hire rates for slaves (which are better at approximating the marginal value to slaveowners of an extra slave) and subsistence consumption taken from the core texts on the economics of American slavery, Jeffrey Hummel estimated that deadweight loss. He placed it at between $52 and $190 million in 1860 with the smaller amount representing 5 percent of total oncome in the region. In other words, the loss in utility of forcing slaves to provide more labor than they otherwise would have had a value of between $52 and $190 million.
But that is not the whole sum of deadweight losses. In the southern states, the enforcement of slavery was not fully undertaken by slaveowners. The states mandated slave patrol duty for free whites. This relieved slaveowners of the costs of enforcement (while they kept the rewards from coercion) which were spread over a large population. The mandatory duty was a tax in the form of labor in kind. In some states, there were actually taxes to finance the patrols. Hummel estimated the sum of enforcement costs brought his estimates to between $64 and $210 million. This represents at most a fifth of the southern economy in terms of inefficiency. This remains a conservative estimate as there was also a deadweight loss from forcibly reallocating non-slave labor towards patrolling which is hard to measure."