I don't know anything about Authority, but if you are concerned about costs I know that Nutro offers a frequent buyer program (you get the 11th bag free I think) and some coupons on their website. Here's a link: http://www.nutroproducts.com/fbpnutroReg.asp
Also, if you shop around your area there might be better prices somewhere. I know around here, prices can vary as much as $15 per bag on the same kind of food depending on where you shop.
Also, I just thought I'd mention, since you said your dogs need to be on a diet, I always have to watch Marta's portions as she gains weight easily. She eats the same kind of dog food as Adele (who will always be skinny) but I just give her less and add some carrots to her bowl instead, they help fill her up, but are a lot less calories. I think it's easier than feeding special food, but of course that works too! Good luck with your search!