Seeing as the dealer is paying Costco a fee, lower is possible. If you contact them through the Costco service, they can’t go lower per the agreement.

I was buying a Honda Pilot last year and tried with Costco first, I ended up buying it a lot cheaper by pinning multiple dealerships against each other.

If you've used the Costco auto buying service, have you been able to negotiate a lower price elsewhere, or is that as low as it can get?