XCOM: Enemy Unknown $17.50 at GMG. Today Only. Req Steam By w00py | December 28, 2012 0 Comment XCOM: Enemy Unknown is on sale for $24.99 (50% off) at Green Man Gaming (GMG). Use the voucher code GMG30-DPLIM-DN831 to save an additional 30%, which makes the game only $17.50. This deal is valid for today only. The game requires Steam activation.