In my searching I've found costco's tires to be overpriced. Tirerack was consistently the cheapest it seemed for what I was looking for, even with shipping. Getting tires mounted at costco is also a colossal waste of time and I'm not impressed with their techs. Just my opinion.
Oddly enough the reason I joined costco in the first place was to buy tires, but that was a long time ago now.