Where I work they have a base fee for grooms of different sized dogs, as well as just for a bath and comb out. It's based on the dog being groomed at home and not matted or hard to to work with. If the dog is shedding and needs ten minutes with a comb before it can be bathed it's one thing, if the dog has not seen a comb for a year and requires an hour of dematting it's another. The owners get told when they bring their dog in if there's likely extra charges. We don't just brush out or demat, owner's policy is the dog gets a bath and full treatment (anal glands, teeth, ears, nails) if they're coming in for a groom.
What she has started doing is if she's charging say $50 for a bath including the extras, and we can't touch the teeth because the dog is untrained or whatever, we spend that time trying or working towards eventually being able to do them and charge the owner the same rate for training. Teeth are usually the thing we can't get done if there's an issue, so we work on it a bit and send the owner home with instructions on how to work with the dog at home in hopes they can return and get it done the next time.
It is trial and error, if the dog is a total nightmare we just don't book them in again. Not worth it.