I tend to CAS planes in the "midgame to endgame" phase. Contact with opponent forces is more or less well established in this phase and you probably are already probing their rear area (or getting you rear probed, excuse my language!
) and getting in close range or engaging ADA assets/units. This is also the stage where many of your arty units run out of ammo. So I use them as a "reserve" artillery force. Many times I don't "commit" them at all.
High cost of modern air units is an issue but not that different from the more general issue of high cost modern "assets" as MBTs, IFVs etc.
As for combat effectiveness (in game terms)... well I don't like "super" units so the fact that planes are not always "spot on" with their cluster bombs or strafe to death a lone crew in the middle of chaos makes the instances where the bombing run goes as planned more "rewarding" as a game experience (or more frustrating if you're on the receiving end).