I'm sorry, but your vet is just wrong. Lantus is now the standard prescribed insulin by any vet who has kept current with feline diabetes.
Humulin N is not a good insulin for cats, but it works well in dogs. Similarly, Lantus is a great insulin for cats, but not so good for dogs. I think your vet is treating feline diabetes the same as canine diabetes, but they are two VERY different things. Cats have much faster metabolisms that require long lasting insulins, while dogs have much slower metabolisms. I've attached an article for you to give to your vet--honestly, I would switch vets. He's not kept up to date, and when you've presented him with new, current information instead of doing his research to see if you're correct he contradicted you without any evidence. If he were my vet, I would demand he show me these mysterious studies that goes against everything that's been published in the past few years.
Sara, I mentioned this in your thread in Health but the pens only come in packs of 5. So the $225 at Target is for 5 pens, not 3. I think the pharmacist misspoke to you or was confusing (It's a pack of 5 3ml pens).
I'm so glad you're switching insulins! You'll see a VAST improvement with Lantus over the N!