You say right in your description that you do not allow pets, @Mon4. With your dog, and thinking of the damage a cat can do especially when thrown together with a dog, I would say no amount of money will cover it. You have every right to say no to the cat.
But my answer to your question is that I charge nothing extra. I'd like to, because there is more cleaning to do after pets, but there is no easy way, and I'm not so adamant about it that I will go the resolution centre route.