Adding dog bite to an insurance policy?

I currently do not have renters insurance, but am looking into getting some (I've gotten several quotes so far and it doesn't look like it would be more than 20-40 dollars a month). I would like to adopt an American Pit Bull Terrier in the next six months or so and was wondering if anybody knew how much it would be to add "Dog Bite Protection" or whatever it might be called to a policy. I know that having a Canine Good Citizenship Award or getting them certified as therapy dogs will lower the amount. But nobody ever says how much.