737-200
Monthly cost: 3,300,000
Monthly revenue: 800,000 x 24 = 19,200,000
Monthly profit: 19,200,000 - 3,300,000 = 15,900,000
Fokker 100
Monthly cost: 2,200,000
Monthly revenue: 600,000 x 24 = 14,400,000
Monthly profit: 14,400,000 - 2,200,000 = 12,200,000
That shows how to calculate the profit a plane makes.
ROI (return on investment) is the amount of money you make relative to the amount of money you have to spend to get that profit.
To put that into practical terms, ROI is the profit you make relative to the cost of the plane.
In the early parts of the game, the investment is the initial lease fee you have to pay to get the aircraft, in the example of the 737, it is 3.3 million. When we compare this to the return, (15.9 million monthly), we see the ratio is quite high.
Now, assume that you bought the aircraft, say it costs 40 million. If we ignoring the costs of maintenance etc., we can assume that you get all the money the plane makes. Therefore, investment is 40 million, return is 19.2 million monthly.
Initially, one might look at this an say 19 million is more than 16 million, therefore buying a plane is better. However, what if we consider it this way - for the initial investment (40 million), you could have acquired leases on 12 aircraft. 12 x 16 million is more than 19 million. This leads us to conclude leasing is more profitable.
However, there are three reasons to stop leasing.
1 - You can't find anymore planes to lease.
2 - You run out of highly profitable routes and can't pay off the leasing costs (this will require you to service a very large number of airports, although it does depend on how many airports are near by)
3 - You have spare money.
Number 3 requires more explanation. The ROI stuff is based on the idea that you will spend all your money - i.e. that instead of paying 40 million to buy an aircraft, you will in fact rent 12. Once your airline becomes very big (assuming there are still planes available to rent), and you are making lots of profit, you will find you do not have enough time to put all these aircraft you are leasing on routes.
So you have the following options (assuming you only have the time to put 5 aircraft on routes): buy 5 aircraft and put them on routes or lease 5 aircraft and put them on routes. Do the calculations, and clearly buying aircraft is more profitable.
In summary, lease aircraft (assuming you make a reasonable profit per plane) until you have too many planes to put on routes, then start to buy aircraft. The worst thing you can do is have money lying around - because you aren't using it to make you more money.
You have to spend money to save [or make] money.
