You mean buying one or upkeep? Because you can get a cat for free, just visit nearby animal shelter. As for upkeep, you need litter box & litter (probably the most expensive thing in the long run), bowl for food, bowl for water and food itself. Then there are vaccines once in a while and once for six months a vial of anti-flea product. Oh, and a scratching post, a single toy (preferably one that has a bell inside or makes other low-key sound when moved) and a transporter (unless you are a fucking cheapstick and use a cardboard box with holes, but good luck stressing the shit out of the animal this way).
So we are talking 30-50 bucks a month here and twice as much during the first month when you buy all the utensiles. And if you don't mind looking around for the best price tag for the litter, then you can save a lot of money this way on the long run. Always buy bigger bags, they are cheaper by default. And focus on the volume of the litter, not the weight of it. Buy it unscented, too, since roughtly every third cat won't use litter box that has scented litter inside. If you will keep it clean, there won't be any odour anyway, while the synthetic scent annoys cats on regular basis and gets tiresome for humans too
tl;dr it's cheap, as long as you only have one cat and litter is the biggest pain in the ass