because a dog is surviving on Purina (meaning Dog Chow formula, not necessarily the entire company), Gravy Train, etc doesn't mean that they are healthy. A human can survive perfectly well living on a diet of Chicken McNuggets and bologna sandwiches but that doesn't mean that they are "healthy". Or that they are in as good a shape as they would be if they ate a better diet.
The cost balance of feeding a higher quality food (don't call it more expensive because it's not necessarily) comes out remarkably even in the end ESPECIALLY if you are planning on breeding. You have a longer life span from a better diet. Less health issues that many people consider part of "owning a dog" such as doggy body odor, shedding and age issues such as arthritis. Also a decreased chance of injury due to better over-all health. As a potential breeding animal, a healthy diet increases sperm count in males, higher ovulation in females (more eggs released = more potential puppies), easier conception, higher likelihood of carrying litter to term and a lower risk of complications during pregnancy and delivery. A healthier female then equals healthier puppies at birth. That equals lower chance of having low-birth weight pups, good milk production in the female, higher quality of milk, and other benefits.
The reason that many dogs seem to prefer cheaper foods is that sugar and artificial flavors are added to make them more enticing to dogs. It's a matter of simple logic - on a basic level every accredited dog food has to meet certain minimal requirements. Many of the cheaper brands achieve this by adding vitamin supplements and chemically enhancing the kibble. Quality brands achieve this by using good ingredients, a healthy balance of ingredients at proper ratios and a bit of supplements to recoup natural vitamins always lost when you cook food. Many also contain extra supplements to help promote health such as glucosamine and others that aren't naturally found in food.
