It depends on the area you live in. Cost can run anywhere from $30 for a spay/neuter clinic that is sponsored by an animial rescue to $300 for large cities. The best thing to do is to just call a few vets in your area. Most clinics are more than willing to give you the cost before scheduling an appointment. Average cost is probably going to be around $75-$125 in most areas of the US.
Please let me know if I can assist further.