There's (at least) a couple of things going on (afaik):
- There's a limit (based on ship strength) of how many aircraft can enter a naval battle.
- There's a bug, such that sometimes land-based air will participate as often as it should (a couple of times a day I think, can't remember) and sometimes will only participate once (full stop) in a naval battle, leading to wildly different results of how effective it is. If you have a naval battle, hover your mouse over the air wing and it'll show when it was last in the battle. Sometimes it refreshes very frequently, other times it won't refresh at all (it'll count up to 100+ hours).
I personally don't find it useless (when land-based air does take part in a naval battle 1-2 times a day, it's actually a tad stronger than it should be, imo, but this seems to be relatively uncommon), but it'll be good when the results of land-based naval air are a bit more predictable.
As for cost - if you plan to have carrier-based naval bombers, you'll need to research the techs anyway. I generally still build some NAVs when I play, but at this stage they're more for flavour.
The word is that Patch 1.4/Oak will have a significant focus on improving air mechanics, so all being well this'll be sorted