If you have more rigs, that means you're paying for more motherboards, CPUs, RAMs, racks and power supplies.
It might be more cost-effective to get more expensive cards.
Actually you can estimate about $750 per rig for non-GPU Equipment.
MB, PSU, CPU, RAM, SSD (Optional), Risers, Misc (wood and aluminum for frame, etc)
Not counting resale value, 1060 is still a better buy than 1070 and about a 10% better ROI give or take.