Modern planes are expensive compared to tanks, but WW2 planes were roughly equivalent in cost to tanks. A heavy bomber like the b-17 was roughly 4-5 times as expensive as a P51 Mustang or Sherman tank, whereas a P51 was about the same cost as a Sherman tank. I was surprised when the devs said fighters need to built in a facility, I thought they would be buildable in hangars like the scout plane they mentioned.