Just let players decide which plane is worth perk points. It can easily be calculated in realtime by plane usage.
first problem to solve: calculate price for each country or same for all? calculating for one country can make one dweeb increase the price for a ride for his country without affecting the others - probably not a good idea. Calculating globally will create a problem when one country upps a squad of 262s (assuming they're very favourite rides players actually would pay for) would increase their price for players of the other countries to launch them to defend.
The solution is easy: Calculate the price for a plane in each country - then charge the same lowest price it is available for in one country in all countries.
To calculate the perk cost, it first is neccessary to calculate the usage of a plane. Let the usage be called U_i,c for plane i in country c. Every time plane i is upped in country c increment U_i,c by F(perks payed)*((total number of planes)-1). Decrease all U_j,c with j for all planes but j<>i by F(perks payed). That way the sum of all U_i,c is constant 0. (be careful with range limits, if one U_i,c is at it's lower or upper limit skip the calculation). The easiest possiblity for F(perks payed) is (perks payed)+1. More stable results might be achived by ((perks payed)+1)^n with n>1 or even EXP(perks payed).
The actual perk cost P_i of a plane can now be simply calculated as a function fo the minimum for all c of U_i,c. The easiest function for this is P_i = 0 for min(U_i,c) <= threshold and P_i = min(U_i,c) - threshold for min(U_i,c) > threshold. A top cut-off might be required to prevent too expensive planes, or a suitably shaped function to e.g. prefere few high perked rides over many low perked ones.
