Yes my cat was diabetic.
When he was 14 he was really ill. He didn’t eat or drink for a week and lost a lot of weight. He was tested but wasn’t diabetic. Turns out he had triaditis (inflammation of three organs - liver, kidneys, can’t remember the other one). He also had a scan and vet said there was something suspicious (tumour?)
He got better, but started losing weight again. This time it was diabetes. I had to learn to inject him twice a day with insulin, and he was put on a special carbohydrate free diet. There’s a place by their shoulder blades, where it seems to be fairly pain free. I can’t remember how I got him used to the injection without him always running away from me. I used to do it when he was eating. It was difficult to find food he wanted to eat, as he wasn’t keen on the special diet food.
I had to take him back to the vets to check his bloods until we were happy that his insulin levels were stabilised.
He stated putting on weight, and the vet said I was over feeding him, but I was sure I wasn’t. Then he had a seizure, and he had to be put to sleep. The vet then reckoned that it was due to the suspected tumour. He was around 16 when he died, and diabetic for about a year.
So, like your cat, it might have been an earlier illness that led to the diabetes.
I can’t remember all the costs, but my cat wasn’t insured.
I used to get a prescription from my vet (£17), which would last 6months?, and buy the insulin online. Much cheaper than buying at the vets. Ditto the special diet. Because he didn’t like it much, I used to check the wet food pouches for carbs, and substitute them now and then.
He led a normal life as a diabetic cat, but it did cost and was a bit of a worry. A neighbour learned to give the injection and would stay and pet sit so we could still go away.
Hope that helps and isn’t too long winded an answer.