Hi and welcome to FDMB. Receiving a diagnosis of feline diabetes is scary and overwhelming. But, like Eliz said, it is manageable on a budget. I know this because at one time I was caring for two diabetic cats, along with 4 others, while unemployed.
To save on insulin, ask your vet to prescribe either Lantus or Levemir. The prescription needs to be for their pens. On both company websites, there are coupons that will bring down the cost to $25 per pen. You get 5 pens in a box. One box of pens should last almost a year depending on your dose.
For food, you do not need to feed Clyde prescription food. It is not low in carbs anyway. Instead feed canned Friskies or Fancy Feast. You want the pate flavors - nothing with gravy in the name.
I also suggest you learn to hometest. This will eliminate unnecessary trips to the vet just to be tested. Those tests are not very accurate because stress can raise a cat's glucose levels significantly. Instead you will use a human glucose meter and test a small drop of blood from Clyde's ear before every shot. The reasons you test each time is 1) to make sure it is safe to give insulin and 2) to determine if you may need to adjust the dose. We can help you learn to test if your vet is unwilling to show you. Don't let your vet talk you into the "animal" glucose meter. The test strips are expensive and the only place you can get them is at the vet. If you use a human meter, you can get strips at any pharmacy. A good inexpensive meter is Walmart Relion brand Confirm or Micro meters. I have also use the Relion brand syringes and was very please with them.
Even though it has been several years since I was unemployed, I figured out the cost for caring for my two diabetics was less than $75 per month. It has probably increase some but still should be affordable.