A blogger created a calculator that allows you to determine (over a 5-year time frame) whether it is cheaper to buy an expensive robot and do automation or to just use manual labor and to save the excess in Treasury bonds. He found that when the minimum wage is at $7.25, it would be slightly cheaper to hire humans, and if the minimum wage is raised to $10.10, then it would be slightly cheaper to hire robots.

The blogger assumes that it would cost $150,000 to build the robot, including R&D and maintenance. This comes out to $30,000 per year (while your assumptions claim that the bots only cost $20,000 per year to replace one human).

http://politicalcalculations.blogspot.com/2014/01/business-m....

I don't know which assumption is correct, though I would guess that eventually bots will be cheaper as we get better making them.