The problem I’m seeing is that your operational cost assesment is slightly skewed because the Japanese has a much larger fleet presence to begin the game with as well as the fact that the US will only make 42 in their initial bankroll as China will likely be in japanese hands from J1. And Japan will be making roughly 35 IPC’s or more per turn by J2.
Also, don’t forget that Japan doesn’t need to match the US on a CV/2 fig basis. As long as there is at least one island or mainland territory in Japanese hands in an adjacent sea zone, Japan can use 4 fighters per carrier. If the Japanese player attacks your fleet it will consist of 2 fighters from the mainland within 4 moves of your fleet that will land on his/her CV and the 2 fighters already on the CV will move 2 spaces to attack your fleet then one space to an adjacent sea zone then one space to land on an island or territory.
8 fig, 2 CV (+1 TP fodder) on attack is roughly equal to 6 figs, 3 CV on defense( I ran a combat sim for this).
Since Japan starts with 6 fighters and 2 CV’s, assuming Hawaii was attacked J1, it will only cost 28 IPC’s to maximize Japans CV potential while it will cost the US (who has 3 figs left after Hawaii is attacked) 78 IPC’s to match that.
My math is thus: Japan has to buy 2 fig, 1 TP(28 IPC’s) to equal the US buy of 3 fig, 3 CV(78 IPC’s).
That’s a 50 IPC difference that would allow Japan to build a bunch of ground units in Asia. And that doesn’t even take into account the fact that Japan has an extra BB and bomber that you will have to spend even more to match. Essentially, 3 turns of US income versus 1 turn of japanese income for both fleets to become roughly equal. 2 turns of income can go a long way for Japan while he/she waits for the US to roll into the Pacific.