Via the Cummings Veterinary Medical Center at Tufts University, "Why do veterinarians prescribe certain diets, such as gastrointestinal-friendly food, and why does it cost so much?"
This is a short article, but it does a very good job explaining what makes prescription diets special. I particularly liked this line: "Typically it’s not the ingredients that make therapeutic diets expensive—it’s the science that goes into putting them all together. "
The thing I always want to add to these articles (and I'm always surprised when it's not included) is the big difference between pet food and human food. Pet food (little beige nuggets/morsels/whatever) has to contain everything a pet needs to live. People buy grocery carts full of food to provide themselves with these necessities, and sometimes we still fail to compose balanced diets for ourselves.
So, developing just a basic, run-of-the-mill pet food takes a ton of time, testing, science, etc. to make sure pets can survive and be healthy on that food alone. Now, imagine the amount of work and thought required when making a major modification to the ingredients of one of those pet foods. For example, cats with kidney disease need a lower protein diet. OK, well, now we have to subtract a chunk of a big, important macronutrient from this food, and then we have to re-test it to 1) make sure the cats can still survive on it as a cat food and 2) evaluate how effective it is at treating the condition. If the food doesn't meet one of those standards, it's back to the drawing board.
Feel any better about how much pet food costs?