How old are you?
From your posts I get the impression you are a teenager (forgive me if I'm wrong!).
One thing you need to consider which hasn't been mentioned is that the cat could live for the next 20 years. So if you are still very young you need to consider how your life could change a lot over the next few years, and think about how a cat will fit into those plans. Also consider what you will do with the cats if you go on holiday etc.
I got my last cat Simeon when I was 18 and had just left home, I moved a few times in the first few years I had him and always had to consider the cat. Then at 19 I had a baby and was a single mother and money was very tight.
You should ring round your local vets and get the cost of neutering, vaccinations, flea treatments, worming (wormings quite cheap at least), and consultation fees in your area. My vet charges a lot less than any of the prices others have quoted so it might not be as much as you think. Our vet does an initial kitten check for free, for instance, normal consultions are £14 and to have macey neutered was only £43. It was the jabs that cost the most, about £70 in total. She also got a free adolescence check when she was a year old.
But if you bought from a rescue centre, as has already been said, initial jabs worming and neutering , especially in males is often already done. In that case the only main you might need to pay for is the second lot of jabs, and the basic equiptment like litter trays, toys and scratching posts all of which can be bought cheaply if you shop around.
Its worth paying for insurance just in case you cat has health problems, but for £8 a month you could have quite comprehensive insurance. For some this is considered a unnessesary expense as most people will never use the insurance, but if money is tight it's probably not worth risking if you aren't able to meet unexpected vets bills.