You could earn different amount of points from different missions depending on how long on average they take. With those points, you could buy more advanced equipment. For example, you could start with a lower level jet (if such thing exists) and purchase something more modern or something for a specific purpose like an A-10 for CAS. This could also work with helicopters and drones. There could also be different ways of implementing it, you could set it up so you buy them each time you want that vehicle or you could buy it once and then if you want it again you could buy the same vehicle at a subsidized price. Just building on my interpretation of your idea.