My persian is supposed to be on prescription food but....?

I just compared the ingredients from the prescription food and they are almost exactly the same as the Purina Pro Plan Urinary Care. The prescription food from the vet is also Purina. The first ingredient in both foods are Corn gluten meal and the only other difference is that one is beef and the other is chicken. Also one has zinc oxide and the other is zinc sulfate. Other than that it's basically the same. But there is a big price difference.