At what point would the deaths have outweighed the rather extreme negative effects of slavery? That would be your answer.
Certainly you could come up with a negative economic value for slavery. Adam Smith understood and wrote about it as early as 1775: "From the experience of all ages and nations, I believe, that the work done by free men comes cheaper in the end than the work performed by slaves. Whatever work he does, beyond what is sufficient to purchase his own maintenance, can be squeezed out of him by violence only, and not by any interest of his own." You could project the costs of purchasing and maintaining slaves, to be passed on to the consumer of the goods produced by their labor, onto a time period of one year. You could also find a value of a human life for one year by assuming a worker's average contribution to the GDP and dividing it by the number of years that the human is able to work. Divide the former by the latter and round to the nearest integer and you'll have your answer.
That seems to me rather like the description of measuring poetry on a page which Robin Williams' character had the students rip out of a literature book in
Dead Poets Society. Create a cartesian co-ordinate plane and place the importance of the poem on one axis and its artfulness on the other and determine the area of the rectangle created. This will give you the value of the poem.
That page deserved to be ripped out of the book. Poetry, like the value of a human life, probably shouldn't be measured this way. The morality which allows one to measure human lives in units of political goals seems as impoverished as the morality which encourages enslavement of men.
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