Since it's more the rule than the exception that your recon plane will get shot down by the OKW buildings a price increase to 80 munitions would be the proper way to balance this. The artillery flare would still be superior to the recon plane in every way. And as Romeo said, a distinctive noise alerting you that they are using this ability would be nice.
Just compare it with the more expensive recon plane, it's loud as shit and you can see exactly where it is flying on the minimap and letting you know that you might have some units in the danger zone. You have no idea where the artillery flare is unless you actually hover the screen over it.
