There are some ways to save on caring for diabetic cats. You may already be doing a lot of these.
1) Insulin - If you are using Lantus or Levemir, buy the pens instead of vials. With a vial, the insulin will become ineffective long before you can use it all. With a pen, you can use almost every drop. Even though the pens may cost more, in the long run, you actually save money. Depending on your dose, one package of pens can last 8 months or more.
2) Hometesting - Save the unnecessary trips to the vets just to be tested. Plus you will know if it safe to give the dose and when an adjustment is needed. An inexpensive, but good meter, is the Walmart Relion brand meters (for US members). The Relion test strips are not expensive. I have also used the Relion syringes and lancets to save money. If your vet pushes the "animal" glucose meters such as the AlphaTrak, they are not any more reliable than human meters. Human meters are tightly regulated by the FDA for accuracy. Even though some of the pet meters may be made by the same manufacturers, I do not know if there are any FDA requirements for pet meters. They are expensive to use and the strips are not easily available when you run low.
3) Low carb/high protein canned food for all of the cats in the household. - this will help lower the BG levels which will hopefully also lower the insulin dose. Having all cats on the same diet reduces the risk of other cats developing diabetes. Fancy Feast and Friskies canned pate flavors are good less expensive food options.
4) Check the Supply Cabinet for items you may use. That can also save you some money.