how much margin does costco get?

Answer 1

According to the Costco Shareholder's Handbook for 2006:

Cost of goods sold: $52,745,497,000
Net Sales: $58,963,180,000
Gross Margin: $6,217,683,000

Source: http://media.corporate-ir.net/media_files/irol/83/83830/reports/70072002.pdf

So Costco's average markeup is about 10%. Some items have pricing restrictions (like alcohol and cigarettes and must be marked up to a certain point by law)