Airline Club

Wait what? You don't renew planes in the used market; you just click "renew" on the plane (or use auto-renew), and the price is always the same based on condition. The renewal price is always (price of the plane new) * (1 - condition * 80%), i.e. you are essentially selling the plane (which always is at a 20% discount to its straight-line depreciated price) and buying a new plane for instant delivery.

(Side note: the actual rewewal price will sometimes deviate a little bit from this formula because of repeated rounding as the condition degrades.)