Don't use constant perk prices for aircraft. Open a free market for planes. Hard to explain what I mean by that, but let me try:
Each aircraft has got a price of X pp (which may be <0 and will cost 0 pp in that case). Every time a player uses that aircraft, increase X by the price for that aircraft (which is >0 for all of them). Every time an aircraft of that type is landed, decrease X by the cost of that aircraft. Do not decrease X below a minimum of -price*(max aircraft in stock).
At the end of every hour (or maybe continuously) decrease the price X for all aircraft by a production value, which is either constant for a type of aircraft, or could be modified by the number of fields owned (more fields produce more aircraft so X will decrease more per hour). Again, don't decrease below a number of maximum aircraft in stock.
This will result in cheaper aircraft for those countries with fewer players. They don't need as many planes, so the cost for them will not grow as much as in countries with more players.
Also this won't allow a players to spawn as many aircraft as they want, because their price will go up when doing so. To destroy enough aircraft of a country to give the enemies an advantage a player needs a lot of pp, I believe that those long time players have enough honor not to do that
