Hello and welcome to the forum! I was in your same position a few months ago and here is what I have gathered. I will summarize the costs at the bottom of this post.
Here is a link to a post where I help a fellow Canadian pick out inexpensive test supplies. You can get the Bravo glucometer, lancets, lancing device, and 100 test strips + shipping for around $60 and it ships pretty fast! This is the same setup I use. You can get 100 more test strips for 40$, and 100 more lancets for $8. Suppose you test your cat 4 times a day, then you are looking at 120 tests. Then add 80 extra test supplies for when you mess up a strip, then you will maybe need 200 tests a month. So that would be 48+48+13 shipping = $109 per month for test supplies (that is all you need). You can get free shipping if you spend $200 on supplies (that is what I like to do). As you get more confident with testing, you will probably find you test less and then won't waste as many test strips, so your monthly cost will go down.
For inexpensive food, you can start with Friskies Pate (I also started with Purina DM but then switched to Friskies). Look at your local grocery store and see what they have and then
compare it to the food in this chart. You want to have <10% carbs. You will want to look for the Friskies Pate (in the US they are called "Friskies Classic Pate" so that is how they appear on the food PDF). Personally, I buy these ones and they are only 60 cents a can.
With how much I feed Dixie (200g a day), this comes out to $24 a month.
As far as the insulin, I get ProZinc insulin from my vet for $114 and the syringes for $30/100. It will depend what type of insulin you put your kitty on, but we are lucky to be in Canada where the insulin costs are lower than the states! (However they luck out with very cheap test supplies). If you give two shots a day, then you will use 60 syringes a month ($18 in syringes). The ProZinc bottle I have has lasted for 5 months so far and is probably nearing the end of its life. So lets say $23/month in insulin. This will all depend on how much insulin your cat needs as well!
Here is a thread that I created to gather info about where to get cheap test supplies in Canada. But since you have a lot to think about right now, I would recommend that you just get the meter and strips (maybe extra strips) from DiabetesExpress, and pick up some Friskies pates. Then talk to your vet about insulin. That is all you will need
I have found that the biggest cost for me is just all the vet visits. If you don't home test and you take your kitty in for glucose curves, then that price will really add up as well! It is cheapest if you just learn to test at home.
So all in all, I spend per month:
$109 for test supplies
$24 for food +
$23 for ProZinc insulin +
$18 for syringes +
=
$174 CAD/month
I feel like the biggest costs have been the vet visits though. The senior blood tests are around $400, and the dental was $2500! And just the general visits for her issues. I will say though, after regulating her blood sugar levels she has become a whole new cat! Many of her issues have vanished (as they were a result from her being an unregulated diabetic).
If you want to get even cheaper supplies, there may be other options but I haven't found much... I bought lancets for half price on walmart (so I got them for around $4-5), but they are much thinner and more annoying and I definitely wouldn't recommend them to a beginner. I have heard other people have success getting things on ebay, but I've never seen any deals in Canada.
You also have other options for the syringes, which depends on which insulin you will get from the vet... so I will hold off on talking about that right now.
Please let us know if you have more questions about anything!