GlobalNow created this Software Testing Automation ROI Calculator that helps provide perspective on the magnitude of the savings and the ROI timeline. The results compare the tangible cost numbers for both the manual and automation approach.
The calculator focuses on the main levers of software testing, including number of test cases, browser support, mobile support, resource cost and the number of product releases over the course of a year. Some other key levers are associated with the process itself, specifically the effective time to execute a manual test and automate a test as well as the rate of change in the application from release to release.
We suggest that you run it at least twice. First plug in the numbers that reflect what you are doing today and second, plug in the numbers that best practices indicate you should be doing. This will help you see the goal posts of how much automation can impact your results.