Mechanic Q: rough idling when heat of air is on?

My car has rough idling. It's barely noticeable if I don't have my heat or air conditioner on. However, when either of them are on, the rough idling is very noticeable. I know rough idling in general can be cause by either a bad spark plug wire or misfiring cylinder. But what could it be in this case an what's an estimate on fixing the issue?