Generally speaking, most "affinity" deals aren't as good as the best on-line discounts.

Thanks, have looked at Drive the Deal, Orangewheels, Broadspeed and Carwow to get an idea of available discounts so unless CostCo are likely to come in somewhere close to those prices I don't really see the point of bothering with their "process" (too much secrecy over the prices, already trapped in a conversation before they reveal).