Having two cats instead of one cat won’t won’t cost that much more. If you use dry cat food, you can buy the bigger bag and that saves you money. You won’t use that much more litter.
The Vet bills will be more expensive but how often you have to take them to the Vet depends on whether they are indoor or outdoor cats.
Indoor cats only need Rabies shots every two or three years I believe and need their Feline Respiratory shots every two or three years.
If they are outdoor cats, then the rabies shots is once a year as they are exposed to other animals.
If your new cat gets along with the one you already have, it’s nice for them to have company when you or your family are not home. My two cats are usually together sleeping on my bed when I’m out or not able to pay attention to them when I’m busy.