On the subject of naval costs going down (so you say), it simply has not. With the one exception of Battleships. I proved it, dont ignore the evidence. You also use a optional technology rule in your argument, not even included in all the games, and is rarely seen. It is not a valid argument regardless, you should move on from it.
Another of your arguments is the 6VC rule for the PTO in G40. One part of one game, that has nothing to do with the cost of units. There are how many other games?
Moving on…
The biggest problem with drastically reducing the cost of naval units, is you then have to drastically reduce the cost of air units, or face a boat heavy game with little air being produced (hurting Germany the most). Then once you lower air cost, now you must also lower the cost of all land units or see massive amounts of air to very little land purchases (esp from Germany).
Then you are back to square one, everything has the same relative cost, just cheaper. This creates a more hectic game, esp for F2F games, and would require more peices, namely chips. And most people wouldn’t accept it.
Air is the Achilles heal to your cost proposals, because air operates over both land and sea, and must be balanced likewise.
From a relative standpoint, the cost of naval vs air vs land is perfect right now in my opinion.
And i have played well over 200 games of axis and allies, many of which competitively. Both revised and post revised rulesets.
I do however feel that just lowering the cost of transports themselves would not break the game entirely.
Lets look at the price system as it stands.
Land: Attack + Defense value = cost (Infantry at 1/2 cost 3, arty at 2/2 cost 4, arm at 3/3 cost 6)
Sea: Attack + Defense X2 = cost (Subs at 2/1 cost 6, dest at 2/2 cost 8, cruisers at 3/3 cost 12, BBs at 4/4 would cost 16…but 2 hits to sink increases its cost to 20)
Air: Attack + Defense + Range = cost (Fig 3 + 4 + 4 =11, Bmb 4 + 1 + 6 = 11…they are adjusted respectively to 10 and 12, with Tacbmb at 11 in G40)
Air is what brings it all together. Without air you have more flexibility in cost.
You could, in theory, simply house rule all units and facilities to 1/2 or 1/3 cost, without breaking the game. You would have to use decimals of course. This would give nations more options when it comes to purchases.