After some testing, which you can find on the discord channel under #feedback-and-ideas posted by me (starcreep48), I noticed that players do not calculate beforehand how expensive a series of flight paths will cost to get from A to B. If A to B requires 10 flight paths, and each flight path costs $1 real money, and their budget is $8, they will fly 8 flight paths and then logout (i.e. they ran out of money). I suggest that a player should know how much a series of flight paths will cost before embarking, sort of like if the player had access to a world map and knew how far it was.
This phenomenon is exploitable and can make serious money without losing subscribers.