Convoys are easiest and most direct way to make capital ships (and the naval game in general) more relevant. They are straight forward and require only a limited amount of stats tracking, since all the values are written directly on the board.
The second easiest way to give fleets a value outside of defending transports or bombarding, is to have subs doing economic damage. There are number of proposals for how this might be accomplished, but the essential idea is that subs can take IPCs away from the enemy in some fashion.
If we lowered the cost for all naval units that would definitely be fun, but bombardment has to be taken into consideration too. If you make cruisers or battleships too cheap, then the effect of bombardment starts to become heavily magnified. I tested the design concept in a TripleA game the Great War with battleships at 18 ipc, and bombardment ended up breaking the set up (this without cruiser bombardment mind you.) So if we did drop the price of all the ships, then the easiest way to deal with that problem would be to just nerf the bombardment value: down to 2 for the cruiser, and 3 for the battleship. I prefer convoys though for easy of use though. They are the simplest to work with and don’t require much additional overhead with the rules.
In AA50 bombers are my favorite naval unit so far :-D