According to my statistics(see attached) only: every ~80 raiders merits a raider boss encounter. Obviously there are more factors but I think they are far to infrequent nonetheless. On top of the 3 I've managed to actually *find *and kill, I've found 1 other and it was already dead.
I'm stuck on that dang contract where I got to kill 5 of these nerds and I can't even find one now that I'm specifically hunting them. (I've been exclusively searching the iridium mine, the spaceport, the mining frigate, and the teraformer.)
Gametime at post - 92 steam hours