Make ALL late war planes that have earlier models in the game be CHEAP perks.
P-51D, F4U-C, 190D9,Spit15 (when it gets in), N1K *when N1k-1j is in*, P-38L (when P-38J/D/H are in), 109 G10, La-7...you get the idea.
Cost? 2 perkies? You fly those planes and you can easily get those 2 perks back and even gain perks, a 3 kill sortie on a P-38L gets you 5 perkies.
Leave the uber rides like tempest and ta152 as they are, expensive perks.
Hey, still no P-61 in AH! Dangit! =P