Hi and welcome to FDMB. It is scary to learn your cat has diabetes, but this is a disease that is very manageable and can be done economically. We all have been where you are now and can help walk you through everything.
The most expensive part of the treatment is the vet appointment to confirm diabetes. They should run a fructosamine test to confirm it. Insulin will be your next biggest cost, but there are ways to get it economically. Ask your vet to prescribe either Lantus or Levemir in the pens. On both of these companies websites are discount cards that will greatly reduce the cost. The pens are better than the vials because they are packaged in smaller containers and you will be able to use almost every drop. With the vial, the insulin becomes ineffective, long before you will be able to use it all. One package of pens will give you enough insulin to last almost a year.
Hometesting you will do with a human glucose meter. A good economical meter are the Walmart Relion brand meters. The meters cost about $9.00 and strips run about $39 for a pack of 100.
For food, there are a lot of non-prescription brands that work better than prescription foods. Many of us feed our cats canned Friskies or Fancy Feast. You want to avoid anything with gravy in the name, pate flavors are lower carbs. Dry food is a no-no. All dry food is high in carbs.