There is no magic formula return depends on City pair, Aircraft, Competition, Route length, Turn time, Seats, Speed.
Getting a return of 100-150k on a B99 is average, allowing you to make a profit in 5-6 game days, or in other words allowing you to lease 3 more per game month on the return the first one generates.
This game is about compounded return on investment and you need to calculate the difference in return a lease makes versus a purchase. As we are a fair way into the game, leases get cheaper. Many smaller aircraft are available in the 10-15% bracket. This means that you can lease 8-10 aircraft for the purchase price of one. This allows you to make an immediate return and you can reinvest this in buying the same aircraft and then replacing your leased fleet.
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