Um...I can tell you right now that cats do not cost as much as a kid, unless you constently spoil them with exspensive foods and toys. A cat will eat about a cup of cat chow a day, which you could get about a months worth out of a single large bag, and you would only need to replace it's litter every week or two, again, about a months worth in a larger sized bag. Other then that, the most a cat really needs are a couple food bowls, a litterbox, a scratching post (unless it's declawed), and a couple cheap toys.
All together, you're looking at maybe 30 to 40 dollars a month.
Of course, there is the possibility that the cat could get injured or sick. Keeping it inside could lower the chance of that, but it's still a possibility and as a person who's owned pets in the past, vet bills can be very pricey.