Whether a Costco-affiliated auto insurance program is cheaper for you specifically depends on your driving record, vehicle, location, and the coverage levels you choose. No single program is less expensive for every driver, and the only way to know is to compare actual quotes for your situation.
Membership-based programs can sometimes offer competitive rates for certain profiles of drivers, but they also come with limited carrier options, which means you may not be seeing the full market. An independent agent can pull quotes from multiple sources so you can compare them side by side. Price is one factor, but coverage terms, deductibles, and what happens when you file a claim also matter.