Do not - I repeat DO NOT - fiddle around with the insulin doses and syringes as flippantly as you are discussing. The U-40 and U-100 syringes are NOT the same dose and you can easily over and under dose, with overdosing leading to hypoglycemia and possible death/seizures/coma, and underdosing leading to possible DKA. Your vet should be working with you to do what is best for the pet and your pocketbook, and if you don't feel they can, then find a new vet. Trust me, we are not making our money on selling Vetsulin to you.
Personally, when I got out of vet school 10 years ago, Vetsulin wasn't on the market at the time (was before and is now but wasn't the couple of years after I got out) and so I am used to starting them on Novolin N or Humulin N. You do not want the R insulins - they do not last long enough and are meant to be used as a CRI in dogs.
Vetsulin was made for dogs so sometimes works better. It uses U-40 syringes. Novolin and Humulin are human insulins, but often times will work fine for dogs. They use U-100 syringes.
Do not willy nilly change the dose of the insulin on your whims either. Glucose curves and/or fructosamines are needed to see whether to change the dose or not, along with clinical signs. Do this correctly at the beginning to avoid complications in the future.
The food is not as important as that they eat two meals a day when the insulin is being given, and minimize snacks/treats. I'd take your money you're spending on the food and put that towards the insulin, which $50/month is not much. Depending on your dogs size and dose, the bottle can sometimes last quite some time.