It is a fantastic and realistic number for Superman given the factors involved.
You're ignoring the fact that, much like every other reboot so far, MOS carried the burden of being a reboot and of being the first film released after a Superman film that left a sour taste for most viewers. Add to that the fact that it only had one or two weeks before it was bombarded with a lot of heavy competition every week after that and the fact that WB practically gave it zero marketing up till April 2013.
Each first film in each CBM reboot will make less money than the average film with the same character would usually make. The Amazing Spider-Man is the highest grossing first-film reboot to date with only $750 million, $100 million less than MOS. Usually it is the success of the second film in the reboot that determines whether or not the reboot was an overall success in the first place. If WB really is dissapointed with the money the film made and expected more, that just shows that they don't know how to run a film studio and how reboots in the comic book film industry work. They gave Superman a small toothbrush and asked him to clean the whole mansion with it in just a few hours. Then when he failed to do so, they were in shock and stretched their heads over it (not the best analogy but you get the point).
Furthermore, it is highly unlikely WB would've left Goyer and Snyder proceed with MOS2 for 2015 even if MOS made $1 billion. Chances are we would have still gotten the Batman/Superman film instead. 2015 will be a huge competition year for films, bigger than even 2012. You've got Avatar 2, Avengers 2, and Star Wars VII. Avatar made over $2 billion, Avengers made $1.5 billion, and there is arguably nothing bigger than Star Wars. These are all properties that even Batman could not defeat at the box office. Same thing applies to Superman. Since both Superman and Batman can't compete with these franchises solo, what was the financial step WB thought would be the best? Have Batman and Superman together in the same film. A very smart financial decision for 2015, but horrible narrative decision.