I would try to double cost of ASF and also giving 1.5x health and dps. This will reduce the number of ASF but you still have enough DPS to stop air experimentals and such. In the moment you build ASF mainly to fight other ASF. You wouldnt need so many ASF only for fighting bombers and air exp. So mainly, even if ASF would cost 10x the ress.
If you can only build half of the ASF, the enemy also has only 50% of ASF, but with same firepower against air-to-ground-units.