The three big expenses for a diabetic cat are food, insulin, and test strips. For food, as others have suggested you can feed any grocery store brand under 10% carbs. Friskies, Walmart's Special Kitty, and PetSmart's Sophistacat all have low carb options that you can get in big 13oz cans to save yourself money. Here's a link to the cat food nutrition charts that tell you the carb content:
http://felinediabetes.com/diabetic-cat-diets.htm
With Insulin, you don't want to look for the cheapest one, because the insulins that work best in cats aren't the least expensive ones. It pays off for both you and your cat in expenses in the long run to use one of these insulins, because they have a very high remission rate in cats (84%) if the cat is newly diagnosed and has no secondary conditions. However, there are ways to use these insulins and keep costs down.
If you get a prescription for Lantus or Levemir, there are coupons for the pens here if you decide to get them from your local pharmacy:
http://felinediabetes.com/FDMB/viewtopic.php?f=15&t=36964. The pens are a bit more expensive up front, but they save you lots in a the long run--because you're getting 5 3ml mini-vials instead of 1 10ml vial. This means that if handled correctly, you can most likely make those 5 pens last a year or more. A vial is only guaranteed to last 28 days, and with good handling some people can get them to last for months beyond that, but more often then not they go bad before you even come remotely close to using all of the insulin. With the pens, you use almost every drop.
I know that a pack of Lantus pens at the Walmart pharmacy is $219 after tax with the coupon. But like I said, they'll likely last you a year or more, which works out to be about $18 a month. I'm not sure how much the Levemir pens are--I hope someone else can chime in on that.
You can also order from a Canadian pharmacy, where it's nearly half the price. Here's a link for where you can buy insulin online from Canada:
https://docs.google.com/document/pub?id=1FCnrPxpYD02Gp5hWtEIbhLXm0VOyaZlAT_qeCjlx-Js&pli=1
Now, for test strips. Home monitoring is going to save you the most money out of anything. If you're testing your cat at home with a human glucose meter, you do not have to pay for ANY vet curves or fructosamine tests. And, it's also the best way to determine dose adjustments and make sure that it's safe to shoot insulin into your cat. The test strips are the big expense with meters, and you want to budget for at least 30 strips a week.
This is an inexpensive, but good quality meter from American Diabetes Wholesale:
http://www.americandiabeteswholesal...nitoring-kit-black_4034.htm?source=SiteSearch
You're getting the meter for free, +200 strips for $45.00, which is about 22 cents a strip. The Arkray GlucoCard is the generic version of the Relion Confirm that you get at walmart.
After that, if you order your strips from American Diabetes Wholesale they are 250 for $46--about 18 cents a strip:
http://www.americandiabeteswholesal...-vital-test-strips_4027.htm?source=SiteSearch
If you're super drained because of the insulin expense and you need some time to save up the money to get the better price on the bigger lot of strips, you can also buy them individually on Amazon.
Here's the meter +50 strips for $23 (free shipping):
http://www.amazon.com/Arkray-GlucoC...ef=sr_1_3?s=hpc&ie=UTF8&qid=1327078831&sr=1-3
And here's the strips alone, 50 for $16 (32 cents a strip including shipping):
http://www.amazon.com/Arkray-GlucoC...ef=sr_1_1?s=hpc&ie=UTF8&qid=1327078831&sr=1-1