The low-cost clinics average about $25 for a male cat, $35 for a female cat, $50 for a male dog and $55 for a female dog. The prices vary widely and depend on weight etc, so check around for the best price. Most clinics will not spay/neuter pre-pubescent dogs and cats. Many animal shelters provide low-cost spay and neuter services.
For a list of low-cost spay and neuter clinics, please visit the Seattle Humane Society's Pet Services website.