RPA ROI is calculated based on the Value of Time Gains. To build a full picture of RPA ROI you need to add two factors into account:
Value of process acceleration (how much actual money was gained by implementing the automation)
Value of error reduction (how much money was saved on eliminating mistakes)
RPA ROI = [(VTG*AC + Value of process acceleration + Value of error reduction) – AC]/AC x 100%
Read more about RPA metrics here