$10 billion. Two separate sources have told me that's how much Google (GOOG) offered to purchase WhatsApp. The bid did not come with promise of a board seat, unlike the Facebook agreement.
Of course, WhatsApp managed to obtain a much higher amount from Facebook who paid $16 billion plus $3 billion in RSUs. Viber, another popular instant messaging app, recently sold to Rakuten Inc for $900 million.
You can download WhatsApp Messenger from the App Store for free.