SteamSpy reports that Civilization V has officially sold more than 10 million units. This makes it the third best-selling PC exclusive in history, only behind World of Warcraft (14.5 million~) and Garry's Mod (13.1 million).
Although Civilization V currently has a $29.99 MSRP, it is regularly sold for under $10 during Steam sales where it has accumulated a large portion of its total population. Firaxis Games is able to sell the game at incredibly low margins thanks to the game's single-player focus; most modern PC games feature multiplayer at the forefront, which requires costly server investment and upkeep.
Civilization V debuted in September 2010 to admiration from critics, but wasn't initially liked by many series fans who viewed it as an inferior product to its predecessor. However, after the release of its first expansion, Gods & Kings, along with free updates, it has been heralded as one of the greatest strategy games in history.
Civilization VI recently released with far greater initial results, delivering a more balanced experience. You can read our review here.