An unfortunate and terrible topic, but one that probably factors into a number of campaign scenarios, especially assigning non-arbitrary costs for ransoming, etc.
The Real World yields clues, and the economics of human labor in antiquity has filled volumes. Leviticus 27:1-7 describes the value of human life in blunt and biblical terms--short answer, ain't much, for all but the most able-bodied of men.
I might look at labor costs as described in the Arms & Equipment manual and assign the wage of "hiring" someone for one year as the rough lifetime value of that kind of slave for that occupation. If that strikes you as too low, two years or five years. I think if there's lots of warring going on, slaves are going to be in full supply and their costs will be low. In more peaceful times, they will be in shorter supply and costlier.
In all but the cruelest periods in history, slaves had the ability to buy or win their freedom / be freed after x years of service / be considered members of the extended household family. Is the World of Elric merciful? Let's hope so.