How could slavery ever become cheaper though? You need some type of guards/supervisors to keep the slaves from just leaving. I guess you could get rid of that and just entice laborers by offering shelter, food and other essentials in exchange for work, but that’s not really the same thing that people generally mean when using the word “slavery” - it’s a 1950s-era coal mining town in West Virginia. Unless someone develops a way to get people to continue working for less than the bare minimum they need to scrape by (i.e. the workers all die of hunger, disease or exposure), there really isn’t a way you can get away with lower cost per worker than by using wage slaves under capitalism.