Estimate savings from open source tools.
Open source is not free, it is differently costed: licence fees fall to zero but migration, training and often hosting or support appear in their place. The honest comparison sets the recurring saving against a one-off transition cost, giving a payback period. Over three years the licence saving usually dominates, but the payback figure is what determines whether the switch survives the first budget review.
Annual saving
Annual saving = commercial seat cost x seats - annual support or hosting cost
Payback
Payback months = migration hours x hourly cost x 12 / annual saving; three-year net = annual saving x 3 - migration cost
Retraining time, integration work with tools that assumed the old format, and internal support burden. These are labour costs that do not appear on any invoice but are real.
Usually favourably. A support contract is typically far below per-seat proprietary licensing at scale, and it removes the objection that nobody is accountable when something breaks.