Check out
catinfo.org for more on feline nutrition and how it affects health, including BGs. For our diabetics, we look for any
foods with less than 8-10% carbs, the lower the better (first Column C). We suggest changing the diet first before starting insulin because this gives you more time to
learn how to home test without the added pressure of making sure the cat isn't dropping too low from both the food change and the insulin. In fact, some cats end up never needing insulin and can go into remission on just the food change alone.
Not sure if you (or the vet) really mean 1000 mg/dl or if he ran some other test, but I've never seen a cat that high before. Usually anything over 600-700 is "Hi" on most meters. Did your vet test him for ketones?
$400 is an outrageous cost for a curve! Usually it's less than $100, IIRC and if you learn how to home test, you don't have to pay your vet for curves since you can run your own curves at home. We also usually suggest starting out at no more than ~1u and seeing how the cat reacts. Do you know what insulin the vet wants to prescribe?
My first suggestion would be to go out and buy an inexpensive glucometer with matching test strips (here in the US, the cheapest is the ReliOn brand from Walmart) and test Sonny yourself to see what his numbers really are. Also pick up some low carb food from the list above and try out a few different brands and varieties to find one Sonny likes. Fancy Feast Classics and Friskies Pates are some popular low carb and inexpensive foods people use.
Transition him to the new food for about a week and go slowly to prevent digestive upset. Continue practicing with home-testing and see how his numbers do with the food change and if they're still high after a week or two, then it might be time to start insulin.