The initial diabetes testing process can cost a fair bit depending on how many tests are needed to confirm the diagnosis and find the proper insulin dosage for your pet. After the diagnosis has been made, there are maintenance visits to make sure your dog is responding as he should to his insulin dosage, plus additional visits as needed if your dog appears to be experiencing issues with his dosage or has developed other health problems. Regular vet care should of course continue as usual.
Your vet may recommend a special diet for your canine friend. A prescription food may be suggested in order to bump up the amount of fiber and complex carbs in the food (which will allow blood glucose in your pet to rise more slowly and steadily, rather than "spike"). A special calorie-reduced diet might also be recommended if your dog is overweight.
For overweight dogs, some people will simply feed less of the same type of food. While this does result in fewer calories, the dog may not feel "satisfied" eating a smaller volume of food, especially if he's accustomed to eating a certain amount every day. Lower-calorie food lets you feed the same amount while also decreasing the number of calories fed.
Insulin and syringes/needles will be your biggest expense. Dogs with diabetes will need insulin shots every day.
Needles should not be re-used. Re-using a needle can cause contamination of the insulin supply and may introduce bacteria into your dog's body. Needles also get dull quickly, and dull needles hurt!
Your veterinarian may suggest testing your dog's blood glucose levels either with the use of urine test strips or with a glucometer. A safe container to discard used needles is also needed. Finally, you'll need to keep Karo or corn syrup on hand just in case your dog starts to show signs of hypoglycemia (low blood sugar).
Follow the treatment plan provided by your vet and monitor your pet closely. Keep track of who gives the insulin shot and when, so that you do not "double dose" your dog. The cost of diabetes treatment in dogs is generally pretty inexpensive and by keeping good control over your pet's diabetes, he can continue to lead a happy life for many years to come.