Aircon uses 8-10% extra fuel when the compressor is running which is not 100% of the time you have the aircon on, this is overall less than the cost of opening a window which more often than not is the alternative. Even in colder times aircon helps esp with keeping windows demised with lower humidity which is why advice is to leave it on auto.
Also not using aircon for extended periods means the pag oil isn’t circulated which can lead to premature pump wear and potentially leaves any moisture in the pipes to sit in one spot & start rotting the system from the inside out.
In short best way to keep overall lower running costs is to regularly use the aircon.