The method is open.
Quoted rate = reference rate × (1 − FX margin ÷ 100). Recipient amount = (budget − fixed fee) × quoted rate. Effective cost in sending currency = budget − recipient amount ÷ reference rate. Annual difference = absolute cost difference × transfers per year.
This simplified model excludes intermediary fees, recipient fees, taxes, funding charges and changing rates unless you incorporate them into your inputs. It assumes the same budget and reference rate for both scenarios. Provider pricing may use other structures. No input is sent to a server.