FYI,
The Box is based on a short story by Richard Matheson called "Button, Button," which was also made into an episode of
The Twilight Zone, as Rbz mentioned.
http://en.wikipedia.org/wiki/Button,_Button_(The_Twilight_Zone)
It's hard to talk about the story or the episode without possibly spoiling the movie, and my response will talk about both, so I'm putting it in a spoiler. Don't read if you plan to see
The Box (or the
Twilight Zone episode or short story).
In the Twilight Zone episode, as Rbz explained, it is implied that the box will go to someone you don't know and that if that person presses the button, you will die. In this case, I would press the button, because whether I press it or not has no effect whatsoever on what the next person does. I could die with the money, I could die without the money, I could live with the money, or I could die without the money. Those are the only options. If I'm going to die, then what I do doesn't matter, but if I'm going to live, I'd rather have the money. Even if it causes someone else to die. Each person's existence is already built upon a mound of sacrifices; what's one more?
However, in the short story, the wife presses the button, and her husband dies, because she didn't really "know" him. The money she receives is from her husband's insurance, reminiscent of The Monkey's Paw. It makes sense, because money doesn't come out of nowhere, and no group is going to hand out cash just to see how people react to a moral dilemma. In this case, I wouldn't press the button, because I'd just end up suffering for the money, and it wouldn't be worth it.
Of course, if someone showed up with the box and offered me the deal, I wouldn't know what the deal was, if it was like the episode or the original short story, but I would take the safe route and not press the button. Also, they would allow me to sleep a little easier, knowing that I was not responsible for someone's death.