Yet another reason not to see Ender's Game in theater: "And Orson is apparently writing something that’s more of a direct follow called ["Fleet School"]. Obviously, from the studio’s point of view, they’d almost certainly want to move the characters from this film into the next journey. So it may be that “Speaker for the Dead” is not the sequel now." Ender's Game director Gavin Hood.

Bigger sales helps Card sell yet another series' worth of movie rights so he clearly does profit if Ender's Game does well at the box office. (I've seen lots of people arguing that he was already paid so it doesn't really matter.)

Source (has spoilers for movie/book versions of Ender's Game):

http://www.slashfilm.com/enders-game-director-says-new-orson-scott-card-novel-may-power-a-film-sequel/