Good idea to adopt an adult cat. They will generally cost less than a kitten because they've already been through their serious of shots so you should only have to take them to the vet once a year for annual check up and any shots they need annually.

One cat is really relatively cheap. Even premium food for one cat shouldn't run you more than, say, $100.00 year and depending on what type of litter you choose, it will cost even less than the food. Then there are toys and beds and condos and such, but you can also get away with that relatively cheap. If you read much here on the cat side, you'll find that most cats prefer home made toys such as crinkled up paper or aluminum foil rather than expensive toys. Instead of a condo, you can put a chair or couch near a window so kitty can see out and towels work well for beds, or if you're handy, someone here has a pattern for making beds which should help keep the cost down. My biggest downfall is walking into PetSmart (or similar type pet store). I need to keep my eyes glued to the floor and ONLY look at what I'm there for - dry food, litter, etc.