Depending on what type of food you get, for an 8-lb bag it will run anywhere from $8 - $13 a month, and assuming an 8-lb bag lasts you a month. Then for a big bucket of refillable clay litter from petco (the one you bring back and refill yourself) I think the first purchase is maybe $15 and then to refill its only like $11 and that will definitely last a month for only one cat.

Then the vet bills but I'm sure you know about that

Overall -- not much at all!