Google shares already lost 4% on the Frankfurt Stock exchange ( http://www.boerse-frankfurt.de/EN/index.aspx?pageID=35&I... ), and Google didn't answer any questions on how they came up with this extraordinary price. I don't think $12.5bn is justifiable...
In terms of what they'll add to Android (patent protection for partners, stronger steering of hardware & user experience), it'll pay for itself pretty quickly.