The cheapest way to get a cat is go to 愛護動物協會, but then why do you want to know about this? It's usually the maintenance fee that's the most expensive such as vet (animal doctor), food, litter box, pet salon (shower, nail clip etc). Also, cats normally can last up to 13 years old or more, so please think about if can really take care of a cat for that long before getting one...