I disagree about the cost being too much. When creating this game, they needed enough dialouge to fill 15 or so planets. Any full expansion would be 2-3 planets and require much less dialogue, even if it is for unique 8 classes and 16 main character voice actors
Well I did hear somewhere that they had voice work done far in advance, enough to last them a few years. I don't know the details. It is only a rumor as far as I know, and hell, I may have read about it in this very thread for all I remember, lol.