Amazon (AMZN) slashed $50 off the price of its 8.9-inch Kindle Fire HD in a one-day "Green Monday" sale. Customers could purchase the 16 GB Wi-Fi only version of the Kindle Fire HD for $249 or the 32 GB Wi-Fi + 4G version for $449. It also cut $50 off the Kindle Fire HD version that comes without built-in advertising. Rival Barnes & Noble (BKS) permanently cut the price of its Nook Simple Touch to $79 from $99. Amazon shares fell 2.2%; Barnes & Nobel fell 7%.