Costco changes their stock and online offerings all the time. HK units may not be offered anymore. You can get a factory renewed HK325 at onecall.com for $399 and a HK525 for $499. Probably two of the best deals around.
I bought my HK 325 through Costco-online a couple of months ago and am very happy with it. Greg is right, Costco changes their stock all the time depending on what they can get the best deals on. I'm glad I ordered mine when I did because they were gone soon after. Maybe you could e-mail Costco and request they try and offer the HKs again? I don't know how responsive they are to online suggestions, but in my personal experience, they have brought in many of the items I have requested (not HT-related products) at my local Costco retail (or should I say "wholesale") store.
Those are great prices Greg told you about at Onecall.com and they are great people to do business with.