I agree with Cook, there's nothing economically nonviable about slavery. How could forcing an individual to work for no compensation be unprofitable?
The most successful enterprises have been those in which the workers, even those at the lowest level, believe that they benefit from the success of the enterprise.
When the workers don't think they will benefit from working harder, or more effectively, the enterprise suffers.
When the workers think that the enterprise is stealing from them - then the workplace becomes an arena in which the workers fight to do as little as possible. The enterprise fights to extract maximum output from the workers, but the added mechanisms to do this drain value.
Furthermore, when the structure of the enterprise depends on maintain ing control of the workers against their will, output level is subordinated to maintaining control. For instance, workers may not be permitted to acquire skills which could endanger the enterprise's control, such as literacy.
Another cost is that to keep severely exploited slave laborers under control, it is necessary to intimidate them with frequent displays of brutal discipline, and for every guard and gang boss to demonstrate their power over the slaves. These displays and demonstrations have costs, in slaves injured or killed. The master/slave hierararchy also requires the masters to suppress or even punish any slave who has consciousness of superior value due to skill. No slave may ever be allowed to be "uppity".
Factors like these are a major reason why the Nazi slave labor system failed to match Allied production levels. It's also why the Soviet gold mines at Kolyma in Siberia had the highest labor costs in the world.
It's also why Robert E. Lee told his sons to avoid having anything to do with black slave labor. It wasn't about the morality of stealing their labor; it was his sense that anything done with black slave labor would be done badly.