Hi and welcome to FDMB. Is Charlie currently on insulin? If so, what kind and what dose is he getting?
There are several things that you can do to help keep the costs of caring for a diabetic cat. The first is diet. You will need to feed Charlie a low carb high protein canned or raw food diet. The vet may try to sell you a prescription diet. They are really not very good for a diabetic cat, most are too high in carbs. Instead, many pate flavors of canned food are lower in carbs. Many of our members feed either Friskies or Fancy Feast pate canned foods. Avoid anything that has gravy such as shreds, bits, flakes, etc. They are too high in carbs. Also, if Charlie is eating dry food, you need to get him off of it as quickly as possible. However, if Charlie is on insulin, we need to know what kind and dose is he getting so we can advise you on how to change him over to a low carb diet. A low carb diet will significantly lower glucose levels and you probably will also need to adjust his dose. Otherwise there is a risk of him becoming hypoglycemic, which is very dangerous. If he is not on insulin, then you can begin the diet change.
I am glad to see your daughter wants to learn to home test. This is the best way to know how well the insulin is working and also will help you avoid giving him insulin when his glucose is too low. You will need to test before every shot and a few times between doses. The order you want to test before insulin is test, feed and then give insulin. You can use any human glucose meter. There are pet meters available, but they all work the same way. The biggest difference is the pet meters are more expensive to use and the strips are not readily available. If you are in the US, the Walmart Relion brand glucose meters are good and inexpensive to use. Your vet may try to discourage home testing or insist on a pet meter, but neither your vet's permission or a pet meter is required. We can help you learn how to test. The other advantage to home testing is that it eliminates going to the vet just to have his glucose levels checked.
Insulin is going to be your biggest expense. The insulins that work best for cats are Lantus, Levemir and ProZinc. Lantus and Levemir are human insulins and you can get them at any pharmacy. Ask for the prescription for pens instead of a vial. The reason is that with the vials, the insulin becomes ineffective long before you can use it all. You will end up throwing a lot of it (and money) away. With the pens, you will use them as mini vials. On pack of pens could give you enough insulin to last anywhere between 9 months to a year, depending on your dose. You can also save money by ordering your insulin from Canada where it is about 1/3 of the cost of what you pay in the US. Also, if your vet does recommend Lantus, ask for a prescription for insulin glargine instead. There are two other brands that are even less expensive than Lantus. They are Basaglar and Semglee. Semglee is the least expensive of the three. ProZinc is a pet insulin and can be ordered through your vet or any vet pharmacy such as Chewy.
Avoid insulins like Vetsulin, Caninsulin, Humulin or any other short acting insulin. They work for some cats, but most respond better to long acting insulins listed above. The short acting insulins have steep glucose drops and do not last as long in a cats body.
There is a lot of information on this site. If you have any questions, please ask them. We are here to help you. Everyone on this board has had at least one diabetic cat, and many of us have had several. Caring for a diabetic cat is not difficult once you find what works best for you. We will help you learn everything you need to know and then some.