This is Ambrose, he's been diabetic since he was 3. Started with excessive peeing and drinking. He went from 14lb to 9lb in less than a month. He would sleep in the bathroom sink to be closer to the water. His is not manageable by diet. He gets 2 units in the morning and 3 at night.
Insulin: I spend $60 once, maybe twice a year on a bottle of insulin (I don't know where you heard $150- that's crazy expensive!). Humulin N is the most common type and you can purchase it at your local pharmacy without a scrip. Syringes are about $4 for a bag of 10 or $30-$40 for an entire box. Purchasing syringes can be tricky (if you need to go that route because diet doesn't work). In TX you don't need a scrip to purchase them, but some pharmacies do require one, I suppose for liability issues. Shop around.
Food: Ambrose was on Science Diet WD for years! Now I have him on Felidae Platinum (more meat less grain) and it works just as well. I buy if from my local feed store for about $23 for a 15lb bag. All my cats eat it- it's like kitty crack. Wet food is the same brand- again kitty crack. You will need to put your kitty on some kind of wet food to be sure he's getting enough water. Even some flavors of Fancy Feast are diabetic friendly. Look for the beef, chicken and turkey kinds- they have less fillers and are mostly meat. I looked into this when I decided to change his food. He was getting too fat on the Science Diet and then he crashed one day and I spent 3 hours and $300 at the vet trying to get him stabalized.
Feel free to PM me if you want specifics about dosages if you need to put him on insulin.
I totally understand about the money. It can be hard sometimes. I hope I've helped put your mind at ease a little bit about the cost. Let us know how he's doing!
:hug: