Start from your actual payouts
Commercial terms vary by platform, country, programme, promotion and contract. The right starting point is your property's actual payout data, not a generic percentage found online.
Calculate the cost per booking
For each reservation compare the amount paid by the guest, your net payout, promotions funded by the property, payment costs and any other fees. The difference shows the effective acquisition cost.
Compare with direct booking fairly
Direct bookings also have costs: website, payment processing, marketing, support and technology. The useful comparison is between the real costs of both channels, not between an 'expensive OTA' and a 'free website'.
Include customer value over time
An owned channel may create additional value by making repeat bookings, direct contact and brand recognition easier. That value should be measured over time.
Use scenarios, not guarantees
A useful calculator lets you change assumptions with your own data. It should not promise guaranteed savings, but help you understand what level of direct bookings could justify the investment.