I don't know, its always hard for me to get excited about buying AAguns regardless of their cost/abilities, probably owing to their long history of sucking in A&A lol. Although I admit I definitely prefer them in Iron War to A&A, since at least here they can move during the combat phase and take territory.
An AAgun (or Cruiser) might make me think twice when it blocks a critical flight path though. So if I know I have to make 2 passes over an anti-air unit to reach a target and return, I might forego the flight plan. I guess in that sense they might be useful to set up lines of defense along desireable flight paths. But with the production slots usually pretty limited, I don't think AAguns would ever top the list for me, even if trying to create a buffer like that.
Fighters and Bombers are definitely my favorite units in Iron War. As Redrum pointed out, they overpowered enough to make most of the other units seem kind of irrelevant, but I dont really mind that for the era. Feels somehow appropriate that the air war would totally dominate and push everything else into the background. When you have this many nations spamming aircraft though, it does leave things a bit open for turn order exploits. Like in that earlier all bomber game I posted, just a few turns spent transiting aircraft to a hotspot like Leningrad or Egypt and you can make it almost unassailable. Even bombers (weak on defense as they are) can put up some pretty kick-ass hitpoint walls that push a territory out of reach rather quickly. Since on defense you can usually have like 3 or 4 nations stacking air hitpoints as a group, whereas the attacker needs to set up a series of 1-2 punches with huge numbers to break through. Some of this is probably unavoidable when we have so many player nations on the board, unless you want to start pulling air units from the purchase roster for minor nations, but then I think they'd become a lot less interesting to play. In A&A style games the can opener play, or the rush air defense play, are usually the only plays that a tiny nation can use to have much impact on the broader game, so nixing their air and its almost like they might as well be neutrals.
I find that after Fighters and Infantry fodder, (+ critical ships/factories to move them around the map), most of the other purchase options are just to spend a remainder. Like if I buy all my desired fighters and infantry and wind up with 13 PUs left maybe I'll buy a tank destroyer, if its 16 PUs I'll get a medium tank etc. Or sometimes its just for flare or showboating, like buying 5 heavy tanks just because you can hehe.
I kind of like the cost structure you have going, even if the fighters a steal, though I guess if the fighter spam seems like an intractable problem you could always create a ceiling on it with pilots as a maintenance resource. Since you already got them in the table for kamikazes and such. Sometimes the endgame can lose its charm when the big dog nations all have double digit aircraft stacks buzzing around. It can start to really limit the playpatterns, so might be worth putting an upper limit on it somehow?