I took a look at the Costco stuff last time I was there from what I recall it's primarily Whey Concentrate, rather than Isolate. That's the cheaper form. From everything I've read Isolate is absorbed much more efficently. Plus for some reason I feel sluggish and lousy on concentrate.

If you buy something at AllSports here at AR, it helps support the site, so that's something to think about as well as the fact their stuff is better than Costco's.