Begin with the delta created by this decision
Record the next level's cost, construction turns and additional income after completion. Exclude existing provincial flow: it arrives without the purchase and cannot repay the same capital twice.
Keep non-cash effects in a separate judgement. Growth, control, recruitment access or strategic position can matter more than direct income, but they do not need an invented universal gold value. Put financial payback beside strategic utility instead of forcing them into one number.
Set a real end point for the calculation
The horizon does not have to be the end of the campaign. A practical review point arrives before the next major war, another army or a credible threat to the province. That is when deployable treasury competes with the building again.
If construction takes several turns, income starts only after the delay. Count productive turns after completion, not the entire period from click to review point.
Run an unexpected-front case
Creative Assembly's official Patch 8.1 notes describe AI priority changes and more dynamic late-game factions. They do not provide a universal probability for your campaign, but they do justify a separate stress case.
Shorten the horizon, add an income-loss risk and inspect treasury after the purchase. If a new front cancels recruitment or breaks the required reserve, lost optionality belongs to the decision cost.
Write the rule before ending the turn
The building passes when payback fits the real horizon, remaining cash respects the war reserve and strategic utility solves the current constraint. Any one condition can postpone the purchase.
Record a campaign review trigger: after scouting the frontier, ending the war or rebuilding a target treasury. That turns the calculation into an operating rule instead of a one-off attractive number.
Replace the baseline with your inputs and get an answer for your scenario.
Data note. Every numeric example is a transparent baseline scenario. It is not presented as official publisher data and can be recalculated in the linked tool.