How much money total would adding a third cat to our two-cat house cost?

Well, initial adoption cost is $100 Approximately how much can I expect the cost to be for spay/neuter, plus vaccinations plus cost of food/litter to increase? plus any other costs i may be forgetting? we currently have two neutered male cats, age 7 mo.s and 10 mo.s. costing us a pretty penny to feed. any help is greatly appreciatd p.s. sorry Im typing like a robot....ILm attemping to acess yahoo from an internet phone, so please excuse random spelling and grammatical errors! i knnow you cant give exact estimates, but a general idea would help!