80% of Costco's profits comes from membership fees (source: http://www.businessweek.com/articles/2013-06-06/costco-ceo-c... ), so Costco is less dependent on changes in buying patterns, and sells pretty much at cost. Unless consumers start canceling their memberships en masse, Costco won't be in deep trouble.

Wal-Mart's profits, I'd imagine, are entirely from markups, and a slight change in consumer behavior (higher prices on gas, higher unemployment numbers, smaller paychecks) makes them a volatile player.