I'm not sure if you are asking about the cost of spaying a cat, or the age at which it should be done.
The cost will vary a great deal depending on where you live. But in the United States, almost every area has a low-cost spay and neuter program available. You can go to www.SpayUSA.org to find one, or ask your local veterinarian about low-cost programs.
As far as the age, it is usually recommended that you have a cat spayed by 5 months of age. They can go into heat and get pregnant at this age, so if you wait any longer, then you need to keep the cat inside to make sure she's not mating.
It is wonderful that you want to spay your cat - this is the right thing to do!! There are about 7 million dogs and cats, puppies and kittens who are euthanized each year just because they're unwanted.
The price to spay/neuter a cat is different everywhere. The best thing to do is call your local vet, your local humane society, and a cat rescue organization to see what prices they can quote you. The SPCA has many vets who offer a discounted rate. In addition they sometimes have "free" days.