What we use is a 10 mL vial of Mylan Insulin Glargine, and her dose is I think 0.03 mL twice a day. That’s about five months or so.
The food is a lot more expensive, at $65 for an 8.5 lb. bag on Chewy. But she’s well worth it. Don’t make this calculation based on “reduced” suffering - your cat can be perfectly fine and happy. There may be some cheaper food options and such to limit the financial impact, but there’s no reason at all that you should give up on your kitty.