[coppperbelle]

The reason you can't find the answer on the internet is because there is really no specific answer.
The cost of dog food will depend on what kind of food you decide to feed.
I saw a bag of Ol Roy dog food at WalMart the other day for less than $14.00. It was a huge bag and would probably last a mediium/large dog two months. It is of course one of the cheapest foods on the market and has basic nutritional value. You will also need to feed more to satisfy the dogs needs. Your dogs poops will also be reflected by the fact that they are eating mostly grains.
On the other hand I feed Solid Gold, Wolf King dog food to my dogs. A 30 lb. bag costs $72.00 plus tax and lasts approximately a month. It is a better quality food, not full of fillers and it not necessary to feed as much to satisfy the dogs needs. Their poops are smaller as they retain more.
I have given you an example of both extremes. There are many foods available in the mid price range as well that your dog will do well on.