Reputed seller talklinesales (99.3% positive feedback) is offering the official Motorola Moto G replacement rear cover, in black, for £3.75.
Built for the first-generation Motorola Moto G. Not compatible with the 5in Moto G (2014) or Moto G 4G LTE. EDIT: We're thinking that this will, in fact, fit the Moto G 4G LTE. See BEEPBEEP123 if you have any problems *winky face*.
(Thanks to jonyfive at HUKD)