Slavery/forced labor is economic in certain circumstances, but only certain circumstances. In many slave agricultural establishments, the work was sufficiently unpleasant that you couldn't get free laborers to do it at any price, certainly not for what you would cost out slave labor at. At some point skilled labor needs literacy and numeracy, which is an issue for the slave owners. The bottom line is that slaves will work just hard enough to avoid physical punishment, and not one bit more. When you start to add incentives, like paying slaves for extra stuff, a lot of things change - although buying some personal luxuries is nice, the biggest reason slaves would accept a money incentive was to potentially buy themselves (and their families) freedom. If that option is not there, then while some money incentive works, its limited. A free laborer is essentially unlimited in their incentive - they can buy a house, open their own business, whatever, so they have incentives to work harder and produce more.
That is not to say that slavery in industry is impossible, but rather that slave/forced labor is inefficient and becomes more so the more skilled the task is. Any slave is valuable, and skilled craftsman or other highly trained slaves even more so. In a south still in the USA, any industries based on slave labor will lose out competitively to industries based on free labor in the north. In an independent CSA, the picture of an economy based on agriculture means a very weak economy.