Disagree for reasons already stated in your other thread.
In essence, making naval units cheaper vastly favors the Allies. Particulars -
“However, an attacker that builds submarines will beat the best naval defense, a fully loaded aircraft carrier, on a IPC-for-IPC basis.”
Incorrect. You make no allowance for fodder units. You’re simply allowing four subs per loaded aircraft carrier in the middle of the ocean, which should never happen unless the carrier controller is either incredibly inexperienced, or just used that carrier to do something really nasty (like giving fighters range to hit a vital target like an enemy capital).
You also make no account for the fact that submarines cannot hit air units. A fighter and carrier combo will almost certainly have carried out fighter attack runs on unescorted subs.
“Even if an attacker use landbased fighters to attack any formation of naval units (including carrier based figthers), the attacker need to spend less IPCs than the defender”
Also incorrect. The fact that the key naval powers have battleships in the beginning of the game makes a big difference. A battleship can soak up a free hit, making any attack prohibitive.
You also have not taken into account the fact that if a landbased attacker simply has a lot of fighters, the attacking navy will be filled with cheap transports. This means two things; the attacker that spent so much on fighters is going to be hard pressed because of lack of numbers, and any attack on the navy will suffer because cheap transports will be destroyed first.
“Any suggestions how to solve the problem?”
Play some games. Perhaps your understanding of ‘the problem’ will change.