Low protein dog food is usually prescribed by a vet as a response to kidney problems rather acute or chronic, kidney stones and congestive heart failure.
Low protein diet is often prescribed, as your pet with a kidney problem may not be able to process the protein effectively.
Expensive option
For most healthy dogs, a low protein diet is not good for a dog. Only dogs with a health issue should feed on low protein 
dog food. After careful diagnosis by your vet for one of the above reasons, he may recommend a low protein dog food.
The problem with low protein dog foods as prescribed by a vet is that such a product can be far costlier than what you were previously feeding your dog on.
It would not hurt to discuss this with your vet. There are alternatives to expensive prescription dog food and in some cases the quality can even be lacking in some of these products too.
Alternatives and cheaper options
Some brands available not on prescription may be of lower protein content but only veterinary prescribed will probably be right for your dog. What you are looking for is around 18% protein compared with around 25% in other brands.
This is a decision you should not really make alone as the health of your dog is at stake but there must be cheaper alternatives. Express this to your vet and see if he can come up with any useful suggestions. Or maybe you can take suggestions to him.
Here are some low protein dog recipes that would be considerably less expensive and take them to your vet for his approval to make sure it is relevant to your dog’s dietary requirements.
Obtain a list of low protein food and brands and seek your vet’s approval. Too often vets simply reach for the expensive prescribed product and he does not give a thought as to how little you may or may not be able to maintain that level of payment to feed your dog.
Obtain your vets sympathy and you just may be able to come up with a cheaper formula between you. It is no joke especially if you have a larger dog that eats more.
Here is another useful site with more low protein dog recipes as well as some insightful comments from contributors.
Low protein brands
Flint River Ranch Senior dog food is a quality low protein dog food. This can be obtained at these outlets at $66.99 for a 40lb bag as well as other sizes.
Another alternative although again not inexpensive is NF Kidney Function from Purina veterinary range which has reduced phosphorous and reduced protein.
Needless to say that these products are a little on the expensive side.
No protein no taste
A common problem with low protein dog foods is dogs do not really enjoy it as protein being reduced seems to take away much of the flavor that comes with it. You may need to mix a little gravy but again everything should be done through consultation with your vet.