The yield of an enslaved person is going to be less than a free person, or the cost to get that yield will be higher (in terms of supervision).
Prior to the current era, people spent a good 50% of their income on food. You can give slaves worse food, but that will be another reason to reduce yield.
At 10 slaves per supervisor, 25% less productivity per slave than the equivalent free person, free supervisor earning 2x what a free person who did the slaves jobs would do (the more skilled the slave, the more skilled the supervisor), and 50% support costs (feeding, shelter, etc) on the slaves, and a 15% per year return on investment (this 15% factor includes slave death, escape, legal issues like a baron taking your assets, etc).
2 sp/day free job produces 0.75 sp/day of value.
0.4 sp/day for the supervisor leaves 0.35 sp/day profit from the slave.
127.75 sp per year or 12 gp per year profit.
At 15% ROI that is 80 gp for a 2 sp labourer slave, or 400x multiplier on the 'free job' equivalent labour.
2 sp is for a healthy unskilled worker. A slave in worse shape will have a much lower price.
