Because it's not much of a budget at all for a sci-fi movie, particularly compared to what Hollywood likes to tell us. Now, for a drama or comedy in which one of the leads is a relative small timer or also has other duties, yes, that's a pretty nice budget-- For example, Clint Eastwood likely made decent bank up front for Gran Turino (and probably REALLY raked it in after you factor it's surprise success in), but the fact that he starred AND directed in it likely costed people less up front than getting a name director AND a leading man with decent drawing power. Even so, the film came in somewhere around $30 million. Note, however, that I said "around" $30 million; that's because coming up with an accurate number for this shit is a pain in the ass. Quite simply, producers and executives lie their asses off about costs all the time. Guys who work for the companies like to show that they can produce great films on the cheap and rival studios like to tell you that the same guys are routinely spending tens of millions of dollars more than they'll admit to the media. Sure, the truth comes out eventually in other ways, but the numbers available around the movie's initial release have a funny way of being subject to change.
