The problem with ASF is not the production. You can double the cost, you will end up with the same amount in the end (will take a little longer)..
The problem is in the air play mechanic : People don't want to use their ASF, because you can lose them on a single battle, and never recover from that. So they form a big blob of them, and do dumb micro for 10 min of full lag at -5.
What that's remind me... Oh, yes, starcraft 2 (minus the lag). Do you want the same to be like starcraft 2 ?
If they were cheaper, but with less HP, that could help (or something like that, didn't put a lot of though).