The Citi card thing is probably big money for them.

k00b

They probably break even. 

Costco has at most 15 percent markup on its goods and services. 

90 percent of their profits come from membership fees 

Bell_curve

Costco's gold bars earn company up to $200 million monthly, analysts say

> Equity analysts said the wholesaler priced its gold appropriately by selling about 2% above spot prices to members before a 2% cash back reward for executive members and an extra 2% in cash back for those with a Citi card.

I realize not all of their customers get either or both of those discounts, but it still seems like they'd take a loss on a bunch of these sales.