Although that sale saw Lenovo pay US$10 billion less to buy Motorola (along with around 2,000 of Moto's patents and an arrangement covering the rest) than Google forked out in 2011, Mountain View reckoned the portfolio it kept from the inventor of the cell-phone were worth the balance.
This ignores a whole lot of details, like the cash assets of Motorola and the separate sale of their set-top box business:
http://www.theregister.co.uk/2014/01/30/google_motorola_mobility_lenovo_sale/