I agree, reducing the cost to 7 would make them cheaper then submarines. Remember, they are still very flexible.
However, I might say:
Attack - 3
Defend - 3
Cost - $10
Move - 6
Makes them even more flexible, and, IMHO makes up for the reduced defense ability.
But then, I’d also say reduce the costs of destroyers to $10 as well. Make a destroyer as good as a fighter in naval battles that way. (AARe already did this, it’s not broken I think.)
I’d also change bombers
Attack - 4
Defend - 2
Cost - 12 IPC
Move - 6
(notice their range was not changed, this is because they defend slightly better now and cost less.)
Likewise, both Battleships and Carriers should cost 18 IPC. Carriers maybe having their defense cut to 1 as well as their attack at 1. After all, the idea was to keep your carriers behind and bring them up for fighter recovery after the area was secured, right? They were not really intended to defend anything, just to bring the aircraft up.
That said, I did once play a game where Japanese and American fighters cost 8. All ships cost 50% of normal. Subs were 4, Destroyers 6, Battleships 12, Carriers 8. That was a fun game!