I worked in upper management there for the better part of 20 years. They do not make the majority of there profit on membership. The average mark up is 15% but some departments are more (clothing). A small Costco warehouse does at least $2.5/week in gross sales. The philosophy is early in early out creating urgency in purchasing. They constantly move the inventory around to create a "treasure hunt" atmosphere. Believe me, they are statistical and merchandising giants. Where else do you go to buy a simple bag of milk and impulsively purchase a leather sofa set? Employees are paid extremely well (over paid for the level of training required imho). In Ontario someone full time at top scale makes in excess of $50k/yr +free benefits + rrsp contributions ($1000/yr) + additional pension matches (another $1000/yr) + 2 lump sum payments a year based on years service (between $1000 & $3000 each time). They are a company that knows how to be successful and can make or break small businesses by simply selling or discontinuing a single sku.
I left the business because retail is a thankless job. Made fantastic salary but wasn't worth it. Working in a corporate environment is not for me, especially at a management level. Staff are generally pleasant to the members because they know early on that it is the members that pay for their homes and luxuries. They appreciate the members. Regardless of how grueling the schedules are or how tough it gets, the staff realize without the members the is no job.
It's a great company but it was certainly a shame to see is former CEO Jim Sinegal step down. I only hope for the employees sake that the current leader can match his compassion and care for the employees.