What is a good inexpensive meat to add to my dogs' diet?

Vegetarian diets for dogs, like the one you are using, still have all the protein and other nutrients that your dogs needs to be healthy. Feeding them meat will only offset this nutritional balance. If you do decide that you want to supplement, ground turkey is cheap, low in fat, and easy to cook. If you buy in bulk, you can freeze little packages and just defrost one a day to mix into their normal dog food.