I would almost always take 3 mech inf over 2 armor
The exception being production space. When producing at a minor factory, tanks are the best buy per slot for ground unit (just not per IPC). Ditto for transporting. Mech are just as ‘bulky’ for transporting as armor but deliver far less combat power on the attack.
For those reasons, armor will still have a role (if more limited in the Pacific).
But CAs just dont have even those saving graces. MAYBE if you are trying to maximize combat power in as few units as possible, but even there I’d rather have a CV and planes than a few CAs.
And as noted above, you cant compare aircraft to ground units for cost because for planes you are paying for the versatility of a four range. But you CAN compare CAs directly to the other ship types since they will be competing for the same role. And in that competition, the CA is clearly inferior in 90% of the cases.