I think the idea that cats are a low maintenance, relatively cheap pet - they don't need walked like dogs and do their own thing - is a bit of a misconception. Our cat is probably a bit spoilt, but he has a routine like a child, and his food definitely isn't cheap! We don't have a cat flap (our choice) so he is in while we are out and always kept in overnight, and when we are in he needs the back door opened and closed for him, although he can go in and out our bedroom window if the weather's nice enough to have that open.
Cost-wise we average out:
Insurance - I know DP paid it upfront, but I think it's about £15 a month
Wet food - Untamed at £32 a month based on one tin a day
Dry food - Royal Canin biscuits, approx £30 a month
Chicken yoghurts (Lick-e-Lix) - about £5 a month based on one a day
Litter - Catsan - about £30 a month
We don't pay into the vet plan mentioned above, so flea and worm stuff is about £50 every three months, and boosters are about £70 annually. Our boy recently needed dental surgery which was not covered by insurance, and that came in at about £800 between the initial appointment, the actual surgery, plus prescription antibiotics and painkillers for the aftermath. We have cat beds dotted around the house and our cat does actually use them all, but many cats prefer to lie on human beds (our cat also does this, of course) and will ignore them.
We have MIL's cat living with us just now and he's on Blink wet food and Hills dry, which I think is probably similar cost-wise to what we have for our lad, but there are obviously much cheaper options if your cat will eat it (ours is a fussy boy!).