Confirming lots of speculation, a source close to Groupon board members tells us that anti-trust concerns ultimately forced Groupon to turn down Google's $6 billion offer. This source says the view on Groupon's board was that a Google-Groupon merger would draw more regulatory scrutiny than any other deal Google has ever done.
Because of this view – that Google-Groupon might not be allowed to go through and that it would take months and months to find out the bad news – board members decided they would need a significant break-up fee if they were to accept Google's offer.
We don't know the number, but our source says the board wanted a break-up fee akin to the one Google gave DoubleClick. A source close to DoubleClick's executive team at the time of that merger tells us the company "got significant protection." In agreeing to acquire AdMob for $750 million, Google also agreed to a $700 million kill fee.