while I agree that Purina (store version) is not high end, their prescription formulation are very good. Theres is a very big difference between regular food and veterinary prescription food. Veterinarians do get decent kickbacks for standard maintenance foods but not on prescription diets. Their are many very important prescription diets that are not available through pet stores because they are only for specific types of cases. I am most familiar with science diet version, but they are all pretty similar.

For instance:
science diet C/D-s (now S/D I think) is used for animals that are prone to forming struvite bladder stones.
C/D-oxl - same but for calcium oxalate based stones.
K/D - for kidney support.
I/D - For intestinal recovery (ie: bland diet)
A/D - appetite diet, for animals that won't eat (very smelly).
Z/D - Hypoallergenic, for animals with very severe food allergies.
etc....

As a former vet tech, I would urge you not to hold it against your vet for having expensive foods or services. In a lot of ways veterinarians do a lot of the things that human medical professionals do at a fraction of the cost. For instance, in a vet hospital a bag of lactated ringers (IV bag) will cost the owner about 20-30 bucks. That same exact bag, from the same exact supplier will cost $200 at a human hospital.

think about this, at my old hospital your animal would get a spay which is full blown abdominal surgery including isoflorine inhalation anesthesia, pre medication, induction anesthetic, pain meds, a full blood chemistry, panel, CBC, PCV, pulse-ox and ECG monitoring during surgery, plus all the drapes, cleaning, sterilization, sharps disposal, medical waste disposal, suture, sterile gauze, sterile gloves, sterile gowns, sterile bonnets, etc.... all usually for under $300. Thats without insurance. I dare you to go to any human hospital and ask to have them stitch up a cut on your arm for less than 500 bucks.

Most veterinarians straight out of vet school (in private practice) make about 40-50 thousand dollars a year, and owe about 80 thousand dollars in student loans. I guess what I'm trying to say is that most veterinary hospitals are not trying to rip you off. They are trying to keep the doors open so they can save animals and put food on the table for their families.

oh, and most modern vet schools have specializations in animal nutrition and a strong focus on nutrition as a part the curriculum. I agree however that a lot of "old school" vets don't put much emphasis on it. that is why it is important to go to a veterinary hospital that has standards (preferably AAHA certified) and not just the cheapest one you can find.