Everything Ive ever been told is around 4-6 months. Ive had vets tell me they wont do it until 6 months, but others will do it based on their weight (Ive been checking as I have a young cat myself who recently decided to adopt me, though he is giving me the illusion that Ive adopted him
). But my boy is about a year and a half (I think) and my other two cats are a neutered male and a spayed female, so no other active cats around to risk getting pregnant. Its all going to really depend on what the vets are going to tell you in your area. Call around and find out what they say.
As for cost, the average around here in Portland area is $110 (pre-op blood work, neuter, post-op pain shot...more for females because their surgery is more invasive than the male kitty surgery). Whoa, just checked your profile info and it says you are from the Rose City too. You can go to oregonhumane.org and they have a spay/neuter coupon you can use (if you have the financial need, as some of us have) or if you need other options...they can help with that as well.