Trainers are passing every penny through to the owners. At $750 - $1500 each month at a trainer, the dog owner is not worried about an extra $35 on the bill for dog food. So back to the previous statement. The pros feed it because it works. For field trial pointing breeds, they like to keep the dogs lean (less weight to carry means they can run faster and further). It takes a nutrient rich kibble to walk that fine line and have the dogs at top performance.
The breeders do not get to pass on dogfood cost. The cheaper they can get litter dropped, and the 8 - 10 week old pup out the door, the better off they are. I am not implying that any particular breeder does not care about their dogs...the good ones absolutely do, but their economics are different.
Purina spends more on testing and long term trials each YEAR than many of the other brands have spent in their history.
I have a cousin that is a vet. He recommends a high priced boutique dog food brand to the yap-yap dog folks, because he makes a nice margin selling it (the brand he sells guarantees that retail outlets pay a higher wholesale cost that Vets do...this lets them sell the overpriced shit a little cheaper that PetSmart. No, I am not naming it.) But he only recommends Prurina Pro to his hunting dog customers...and he does not sell that.