Tin of cat food is like 87p - that feeds my cat for 2 days. Litter is dead cheap and lasts quite a while if you have an outdoor cat. Toys are like £1-10 depending on what you get.
Vets depends where you go, I got my first consultation free but the anti-biotics were £30... but that has been a one time charge in the past 10 years. You don't really look after them, they kinda do their own thing and on an evening they will come by you and just lie down - as they age they sleep a LOT more. Monthly / 3 monthly check-ups are the most expensive part for us, but it's nothing that steep.
You wont be able to wear any black items without having some hairs on there lol, unless you keep the cat away from the clothes and washing process but I doubt that's even do-able . You can get lint rollers from Poundland to get everything off before you go out, but accept that having some white hairs on you clothes will be a given (others don't notice anyway, all people with pets experience this) Expect to get fleas some time in the next couple of years - even if you use the flea products regularly, this just seems to be a common occurrence to every cat owner. Flea the carpets and her if you notice s/he itches often and regularly use a flea comb to make sure they are not carrying any. Some like baths, others don't. Some like collars, others don't. Some like playing with toys... again, others don't. Try see what personality your cat has and don't force them to wear a collar (especially a bell one where they can get anxious).
Sometimes they fight other cats in the garden, and depending on their personality might need you to rescue them. My 11 year old does this a lot and she isn't a fighter. If it's a girl then she might get pregnant randomly, if you choose to keep the baby then do NOT touch them for a few weeks (read online about how to look after kittens) or she will kill them. My cat killed 4 of her kittens after we touched them too early. This is because of scent.
Young cats will run around a LOT in the house when you first introduce them from the shelter / past owner. I have no idea why but this used to scare me when I was a kid. They might also pee in corners / behind chairs....
Finally, I would like to say a lot of this is daunting to read, but honestly having a cat is the best experience ever. They are adorable, little family members and although they might not like you at first, it is important to stay in the same room with it (leave for 30mins or so to give it a chance to explore) so they can know who you are.
OH and they love soft scratches (scratch the fur not their skin) around the face - this is because the scent glands are here!
I hope this is readable haha, I just typed everything I can think of down...
edit: I also tried leaving out the "personality specific" things because I know every cat is different, that's why I talk so much about costs and stuff lol - there's a lot more you will learn by experience