A slave also needs more management. Migrants and free workers will get themselves to the job and in general work. A slave has no motivation to work harder so you need some form of "slave driver" to keep them working. If you try to move your slaves around like migrants move, then you need a manager to go with the slaves to keep them working - migrants manage themselves.

A slave is cheaper if you have a lot of repetitive, low-skill, year round work that must be done by hand. However most of that type of labor is easy for the industrial revolution to automate.