One thing to consider is the quantity of food needed to satisfy your dog..If you feed a lower quality food with lots of corn and crud, you may need to feed more food than with a high quality food with less "filler". Compare some of the feeding guidlines and you will see what I mean. I have found a higher priced food to be more economical because my dogs eat less of it. Stool size/quantity is also less because they are able to digest the food better.

Its worth it to spend a little time researching what is in your dog's food. You may be really surprised and grossed out by what you find.