It's a job, and pays just like any other job.
OR
If they dont get payed at least they get roof top of their head and probably food, and you never know if you find friend within the corps.
OR
You're dumb thug who knows nothing except how to be thug, so you joined the Sith Corps to thug it away while they provide you food so you dont die.
OR
They're power hungry people trying to climb the "corporate ladder" in Sith Corps, so they would finally make it to Captain or sum such.
And the list goes on....
Basicly, look at any real life motivation to get a job and you've got it, I think you can't generalize Sith Troopers and their motives, they're individuals even though the world is fictional.