Well, if you need more justification for Costco membership, here's where they're cheaper than the commissary and exchange that I've seen:
film developing digital prints developing electronics small appliances printer paper DVDs and CDs books batteries in large quantities (necessary for Game Boys) diapers baby wipes fresh flowers lg quantities of paper plates, etc. (necessary for parties) clothes (not much selection but excellent prices) good quality towels name brand toys hot dog and hamburger buns (lg quantities are needed for bbqs) sheet cakes (delicious and very cheap) freshly baked muffins, cookies, croissants, etc. party platters some frozen items
Plus they have some things not available on base, such as rack of lamb. Just wanted you to know what I've priced so you can decide if it's worth the membership!