Always found the success of Cosco to be wild. Here is a quick article on the Cosco Wholesale Club. Part of the Kei Club over at Zulit.com
Here is some info from the piece.
Cosco Wholesale Club Model of Business
Cosco focuses on selling products at low prices, at high volume. These goods are usually bulk, and marketed to large groups and business operations. Furthermore, Cosco doesn't carry multiple brands or varieties where the item is substantially the same. This results in high volume of sales from single vendor, allowing further reduction in price, and reducing marketing costs. Cosco also saves money by not stocking extra bags or packing materials; to carry out their goods, customers must bring their own bags or use the merchandise shipping boxes from the company's outside vendors.
Membership fees at Cosco are 50 dollars a year for Goldstar and Business Memberships, which can be upgraded to Executive membership for an additional 50 dollars a year. Along with the additional benefits the executive membership has (e.g. car purchasing savings, home loans, car insurance, check printing services) executive members also receive an annual "2% Rewards Check" from Cosco on all purchases made (excluding gasoline and tobacco).