Don't know about the US but for the UK the Civ 5 Expansion Pack Gods and Kings is available on GMG for £2.10 which is a bargain and it's a steam key.
£2.99 http://www.greenmangaming.com/s/gb/e...-kings/?sale33 + Use voucher GMG30-DPLIM-DN831 to bring it down to £2.10