I agree. They could be making $100 million in revenue a month but if it costs them $95 million to make that then a $21 billion valuation seems crazy on a company that profits $5 million a month.
No offense, thinking about it is fun, but we're not exactly accountants; and we're definitely not accountants with access to Groupon's numbers. It "seems crazy" isn't exactly a critical argument.
Complete vanity.