Japanese planes had good range and weren't overly sophisticated or armoured, they didn't change the Zero much throughout the war so that would probably have good economy of scale as well.
The Il2 was one of the most produced aircraft during the war, but probably didn't cost much for the Russians either, but then slave labour is cheap and their aircraft were obviously suffering if you consider the reputation for shoddy build quality many russian aircraft have (Germans also used slave labour and probably the Japanese as well.)
Britian had some problems with strikes I believe so that would have affected their cost of production, US labour probably wasn't cheap either, thats another consideration.