Well, yes and no. Most runners are definitely looking out for number one, but they might do that in a couple of different ways. Sure you could rip off the rest of your team, but then at the very least nobody would want to work with you. At worst, you end up with a severe case of lead poisoning. There's a lot of thieves' honor in the shadows, because if you screw people over, it's not gonna stay a secret for long. And if you let the mage take a bullet instead of saving them, you might end up with an angry spirit on your hands and nobody to banish it. So you'd be looking out for number one by looking out for the rest of your team.
That's assuming you get caught, of course. If you're the only one who knows how much you're gonna be paid for a run, and you keep half of it, it's not a given that somebody will find out. You might get away with it.