If you think the cost of a carrier and a pair of planes is too much for the United States, imagine how bad it is for Japan! The US has no distractions or need to defend land, just help out a bit in Europe and crush Japan with their uber economy. Japan’s got to deal with Australia, India, Russia, China on the ground and Australia, India and the United States at sea.
I’d say drop the cost of carriers to 10 IPC, fighters to 8 IPC and tacticals to 10 IPC, remove the silly NO for not losing any of the 48 Continental US States (if you lose that, you’ve already lost the game, having it in is just pointless.) Replace it by making Hawaii 5 IPC, Midway 3 IPC, Guam 2 IPC and Wake 1 IPC (and make the NO for Japan to control 3 of those 4 island groups.)
Net effect on the US? None. They have the same income and, in fact, get it for a few extra rounds since Japan will probably NOT be taking all those territories in round 1. Even if you gave them a 50 IPC bid, I doubt they would take all 4 territories in round 1.
What it does do is give Japan an chance to take those IPC away from the US. Also by reducing the cost of carriers, etc, Japan’s got a better chance of replacing their carriers and thus putting more risk on the Allied fleets at sea. Likewise, Australia/India might actually be able to afford a carrier AND something else in a round.
Fighter/Tactical price reductions will help out all across the board.
These are just my opinion, untested, off the cuff.