I have just stumbled across this forum and felt the urge to reply. Vets and nurses are required to undertake a minimum amount of continued training every year so they are indeed kept up to date as you like to put it in all fields. True nutrition is a small part of the course however once in practice it is a major issue and one which is covered greatly. There is no need to have your animals on "the most expensive diet" as you put it as mine are not however there are good diets and bad diets just as there is good food and junk food for humans. I would like to clarify a few points;

Dry food is much better for teeth, it has not been proven to clean teeth and is not recommended in this manner. Dry food works the teeth and helps prevent the build up of tartar whereas wet food tends to cling and encourages bacteria build up. This is based upon animals in the wild who would eat the whole of the prey bones etc included which would work at the teeth.

70% of wet food is water.

The reason "supermarket" brands are not recommended is not because the vets get kick backs but because they simply use the cheapest of ingredients rather than sticking with a standard recipe. This means each tin/bag is different and can actually cause stomach upsets and other problems.

The majority of the price you pay in supermarkets is marked up profit from the supermarket. Everywhere has to make a profit or they would no longer be there.

If at the end of the day you do not want to switch foods ask your vet nurse to design a feeding plan around to foods you would like, most will be happy to do this as they want what's best for the animal.