Green Man Gaming offers downloads of Sid Meier's Civilization V: Brave New World for Windows and Mac for $29.99. Coupon code "GMG30-LY2G6-UKDYY" cuts it to $21. That's the lowest price we could find by $9, although we saw it for a buck less five days ago (since expired). This very well-reviewed expansion pack requires Civilization 5.

Note that this game requires a Steam account to install and play.