List costs and benefits separately
Keeping every line item visible reduces the chance that a convenient assumption disappears inside one total. Include implementation, operating, transition and opportunity costs when they matter.
Align time periods
A monthly operating saving should not be compared directly with a one-time implementation cost without defining the analysis horizon.
Use scenarios for uncertain values
Best/base/worst cases are more informative than pretending one forecast is certain. Document why a number changes between scenarios.
Keep non-financial impacts visible
Not every benefit belongs in dollars. Safety, quality, compliance, customer experience and strategic fit may need explicit qualitative notes beside the financial result.