It has been about two years now since I have had to buy these things, but the last time I did, I was spending about $25 for the vial of Insulin, about $28 for a case of 100 syringes and about $16 for a 4 pound bag of a prescription dry food - this all lasted about a month, sometimes more or less, depending.
There are also vet fees to think about ... and unless you are willing to test your cat's blood at home with a glucometer then you will need to take her in at least once per month for a sugar check - after a while, I got smart and requested the serum fructosamine test instead. It gave an average of my cat's blood sugar levels over a two-week time period and I found it much easier to regulate using the data it gave us.
If your kitty is newly diagnosed, your vet fees may be higher at first but will decrease significantly once you have her better regulated.
One thing I want to mention to you is about the prescription food ... most vets are still recommending the Hill's Prescription Diet w/d for diabetic felines and mine all ate it - but I did a little research and found that Purina also makes a diabetic food formula (Called Purina Veterinary Diet d/m) and so I asked my vet if we could try it for the last sugar kitty I had ... she started on this food the first week of October and by the first week of January, her insulin requirement dropped by 4 units per day. This was an amazing discovery and it really helped to control her diabetes. You might want to ask your vet if you can try it - while it may not work as well for your baby, I had excellent results with it.
Here is an article which was published in a recent issue of the TCS Newsletter - it may be helpful to you:http://www.thecatsite.com/Cats/Cat_H..._Diabetes.html
Best of luck,