Pax spend in the terminal: Duty free, coffee shops, newsagents, restaurants, travel agents, foreign exchange facilities, entertainment centres.
Pax spend to//from airport: Car hire, bus, taxi, car-parking
Auxillary services spend: Servisair, Fuel, airport security, customs
But very little of that actually goes to the airport operator - nearly all of it goes to the franchisees. MAG does not run most of these trhemselves. So that approach would do VERY little to pay back MAG's investment.