Again, there is no such thing as a "Costco price.". Membership with Costco "allows" a member to utilize their referral service.
Let me break this down for you as someone who has worked at a dealership that was part of the program. If we received a lead/referral from Costco instead of sending the customer a price quote via email with our dealership's standard email formatting we sent the exact same email with the exact same price quote with Costco trademarked stationary. We want to put it in the customer's mind that this is some special, exclusive deal that they are getting because they have a Costco membership and Costco promotes the program in the same way as well.
If that gets people committed to purchasing a vehicle without any fuss than it works out well for all involved parties. Costco gets a paid membership and the member will want to patronize Costco to get what they paid for with their membership, the dealership gets at least one sale from that member, and the member believes that they got a deal on a vehicle that they would not have gotten otherwise.