Final Fantasy XIII-2 Fails To Cross 500k In North America
Four months after its release in North America Square Enix’s sequel to Final Fantasy XIII still struggles to surpass its predecessor.
As of May sales of Final Fantasy XIII-2 had not made it pass the 440k mark, according a source familiar with the matter. In the US the PS3 version of the rpg accounted for 63% of FFXIII-2′s sales while the 360 version made up 37%.
In comparison Final Fantasy XIII sold over a million copies in a month when it was released back in 2010. Worldwide sales of FFXIII secured its position as the fastest-selling title in the franchise’s history.
Final Fantasy XIII-2 saw better sales in Japan where the game has sold 800k. Exact numbers in Europe are unknown but sources say the rpg sold the least in that part of the world.
Data courtesy of Erren Van Duine, Senior US Editor for RPGSite.net.

so?
side games never outsell main games