Yes, Costco is a little cheaper....but there is a hidden charge from what I hear.
Its kinda like Sam's Club (if you know anything about them). The prices you see are for members, you can join for sometimes only $40, but that is also about the savings for some things, so why bother.
Anyway I have been looking into the Southern California City Pass & it is cheaper through Costco then anywhere else. Even w/the 5% fee for not being a member, it is still cheaper.