Time saved is not money saved
Most automation business cases open with hours reclaimed. Hours only become money when a role is not backfilled, a shift is not added, or the same headcount absorbs materially more volume. Start the model with the cost line you expect to change, then work backwards to the process.
Model four numbers, not forty
Volume of the task per month, fully-loaded cost per unit today, expected automation containment rate, and cost per automated unit including model, telephony and maintenance. Those four numbers produce a defensible payback period. Everything else is commentary.
Price the failure path
Every automated workflow escalates some share of cases to a human, and escalated cases cost more than they did before because they arrive later and warmer. Include that penalty explicitly at the containment rate you actually expect in month one, not the rate you hope for in month six.
Instrument before you deploy
If the baseline is not measured, the improvement cannot be proven and the programme loses its budget at the first review. We insist on two to four weeks of baseline instrumentation before a single agent goes live.