I was told the same about the 600 from Costco as I just ordered my Denali a few weeks ago. The program is only good until the end of the year. Costco may have another program just as good for next year.
You can try Truecar.com. The Denali I ordered was 74.6K MSRP.. Supplier was $69.4K Truecar came out at 68.5K. The dealer honored the Truecar price.
Interesting, the MSRP is showing around 76190 and supplier is coming out 70864 which the dealer implied they would honor. (I will know tomorrow)
Checking Truecar right now, I see 73797 as the target and 72279 as the average paid. Seems like truecar is higher that supplier pricing around these parts.