Costco only carries a few choices of particular item, then sells in bulk packaging where even the shape of the box has been specifically tailored for Costco so they can get it shipped from manufacturer as cheaply as possible.
IIRC, their margin is something like 15% (even slightly less than that on their own brand), and they make most of their profit from renewing memberships and ever increasing the number of members at ever increasing new warehouses opening in U. S. and in other countries. Super market markets are about 25%.
Walmart has to stock so many different items (100,000), and many different brands of particular item. Costco only stocks 4,000 items.
I kind of suspect that even though both are retailers, the Walmart and Costco business models are an apple to oranges comparison in terms of how they make profit.
http://www.hulu.com/watch/368405 (start around 6:15 mark; average salary of $20 / hr, plus benefits (health for 90% of employees is around 19 minute mark. I assume that this is for their full time employees, a job which is very, very hard to get because of pay and benefits.)