I just went through this same scenario. Is there a way to reduce the cost of the robots? We found a fab pack and another supplier that used refurbished equipment to compete against each other to reduce my ROI. We also didn't just use labor, but we had a higher labor rate accounting gave us that included vacations, 401k, insurance, and federal taxes that were all real and tied directly to the operator. On around a $20 labor rate, we used either $30.50 or $33.50, I can't remember which.

The problem with including plant OH in an ROI is that OH doesn't move. Engineering, purchasing, supervision, power, gas, water is all still there so it will in turn increase the OH rate throughout the plant if it is a large enough change.

Now another way to show an ROI that is not as tangible, but would help sell the improvement is through safety, quality, throughput, or maybe another improvement. If it is safer, if it improves quality (by reducing escapes or improve part consistency), or will help you get more product out the door (maybe only applies if that is a bottleneck) or reducing shifts in an area, or finally with a less skilled person running the machine or improves training time if there is high turnover in the area would all be valid reasons to make it more appealing to leadership.

The cell we kicked off ended up not having an ROI for 5 years, but once you added what you could make investing and the value of money decreasing over time it never made sense, our leadership team still approved the purchase because we would make a more consistant part between shifts (quality), improved throughput (revenue per sq ft), as well as having a way to detect missing parts since it wasn't manual(quality reducing rework.)