Lots of good advice, here. I do want to share a few words of caution, though. Unless your dog is very active and is building/maintaining muscle, do NOT overload on the protein. Too much (unneeded) protein is hard on the kidneys. Very active dogs (just like humans) need the extra protein, for muscle building/maintenance. Couch-potato dogs do NOT. The average dog does need adequate protein, of course - just not an overabundance of it! I found this out, the hard way, when Phoebe developed a kidney stone (many years ago, when she was young and on a "premium" diet). Since then, she's been on a lower-protein diet, has plenty of energy, shiny coat, and is as healthy as she can be. Now at 10 years old, she's still a lap-dog, at heart . . . but has more than enough energy to run and play. Anyway, because of that, I started really researching the protein issue.
Another interesting fact about high-protein - it can also cause some dogs to be more aggressive.
Corn is a problem, IF your dog is allergic to it. Otherwise, it's an excellent (and inexpensive) source of carbohydrates. Just because it's cheap, doesn't make it bad . . . unless your dog has a problem with it. Many dogs thrive on foods with corn as the first ingredient. Potato is another excellent source of carbs, as is rice.
Many people argue that dogs are carnivores and should primarily eat meat; in reality, they eat whatever is available. In the wild, their ancestors ate the whole kill, not just the muscle/meat. The whole kill included the stomach and its contents (most often of herbivores), thus supplementing their diets with whatever the other animal had eaten. So you can look at the teeth (which were necessary for making a kill and tearing it apart) and see a carnivore . . . but if you look at their actual nutritional needs, you'll see an omnivore.