A one-page method using hours, hourly cost and error rates, with a worked example.
Count the hours
List the task, the people who do it and the hours each spends per week. Be conservative.
Price the hours
Multiply by the loaded hourly cost, including benefits and overhead, not just salary. 25 people spending 6 hours a week at $45 an hour is $324,000 a year of time.
Compare with the full cost
If 40% of that time is automated, the saving is about $129,600 a year. Set it against the build fee plus running costs. If payback is over 12 months, pick a different task.