Having a diabetic does NOT cost $150 a month. This one you can totally trust me on, because I had a diabetic cat for five years, and I run a group for people with diabetic cats. So I know what I'm talking about here.
As for putting her to sleep... I'm not going to beg. I'm going to be honest. Would you put a human to sleep because they got diabetes?
Some quick suggestions on cost:
First, test blood sugar at home yourself. It's a heck of a lot cheaper than going to the vet's to do this. It's also more accurate because stress raises blood sugar and cats get stressed at the vet's. You can get a meter free with a rebate or pretty cheap. You can even get testing strips on eBay (as long as you watch the sellers and expiration dates carefully). It's good to test often, but you can minimize the testing if you seriously can't afford that many testing strips. But you certainly can't beat the price if you do a curve at home instead of at the vet's.
Second,
you absolutely DO NOT need a prescription diet!!!!!!! I cannot stress this enough. Diabetic cats (all cats, but especially diabetic ones) need to be on a high-protein, low carb diet. Friskies canned ground-style will do, if that's all you can afford. Yes, Friskies canned. There are better foods, but the prescription stuff you're getting at the vet's is not any better. (The first Hills food to show up on that food recall was MD, their diabetic food. Get it? It's not any better.) And Friskies would be better than the obvious high-carb dry you've been feeding. (Likely the reason she became diabetic in the first place. Don't take that as an insult. I ended up with a diabetic too, remember? ;) ) The prescription food is not any better than the basic ground/pate style canned stuff you can get at the pet store for a lot cheaper. And in fact, the prescription stuff is higher carb than the canned junk from the store! Fancy Feast pate style is a good choice, cause cost is reasonable, it's easily available, it's low carb and most cats like it. It's what a lot of people with diabetics feed. Just don't get the Science Diet canned, cause it's even higher-carb than their prescription food! Your vet learned feline nutrition from pet food manufacturers, who teach vets to feed their foods, and it never seems to occur to vets that they have an ulterior motive. Sigh. Or, they are just trying to make more money off it, at your expense. (And the expense of your cat.) At any rate, seriously, forget the stupid prescription food! (My cat was diabetic for 5 years, until he died of cancer, and I never fed him prescription foods.)
Third, you can probably get the insulin online rather than from the vet. I don't know what was prescribed. And cats also need very tiny little doses, so you won't go through it very fast.
And special treats? Huh? Your cat doesn't need treats at all, let alone special ones.
Please listen to this next part closely. Treating a diabetic will take more money than not having a diabetic at all. However, not only can you keep costs down, but if you start now, feed the right food, test the blood sugar at home, and learn what you are doing, your cat could actually go into remission. That is, the diabetes could go away. It's not like I can make a guarantee of this, but if given the right food and following the right procedures with the insulin, there's a high success rate for this. Part of it is the food, and the rest is a lot of complicated stuff about the pancreas and liver and such. If you do what you need to do now, you could end up with a cat who no longer has diabetes. And the only thing necessary to keep the diabetes away should be to stay on the low-carb diet. This isn't BS. I'm online dealing with this issue all the time and I've watched lots of cats go into remission. The people online are better at it than most vets.
You're welcome to PM me to get more info. Feline diabetes is my forte. I also urge you to go to:
Feline Diabetes —Diabetes in Cats — Treatment and Diabetic Cat Info — FDMB\
It's probably the biggest, best site out there for feline diabetes. There's a great forum there too. (That's where I originally went and learned, though I don't have time to be there anymore since starting my own group.) There's actually a lot of resources out there, but that's a good starting point and has a lot of links. It also has an article about "frugal diabetes." This site alone will be enough to overwhelm you, let alone me giving you a ton more links. :p