The machine still has an equivalent annual cost to compare to the workers salaries. Some things that you now have to pay for with machine labor compared to human labor below:

1. Up front payments: Parents and schools subsidized the manufacturing cost of human workers. With machine labor that cost is passed on to the buyer.

2. Energy: Employees feed themselves via their salary, or is often the case with subsidized food stamps. With machine labor the cost of energy falls entirely with the owner.

3. Replacement: If employees die or are otherwise unable to work do to catastrophic events, they can be replaced immediately with little on-boarding cost. If machines fail catastrophically the investment is lost(or needs to be backed up with insurance). Also, on-boarding the new machine requires specialized higher wage human labor.

4. Repair and modernization: As machines age they incure greater upkeep costs. They also become obsolete by newer machines. As employees age and become less capable of performing their duties, they can be let go, free of charge, for poor performance and replaced with a younger worker.

Much like was discovered with slavery and indentured servitude, ownership of labor and free labor can be fairly close in cost if you know how to really exploit workers effectively. Harvesting in agriculture is an interesting place to look at how competitive exploited human labor can be compared to machine automation.