Whether or not you think 400 points is too much to spend on 90 minutes of additional gameplay depends on how much you're interested in the Mass Effect story. If you want to immerse yourself in everything the Mass Effect universe has to offer you, including the novel, then you should definitely be downloading this episode.
If, however, you're dithering and you're not sure, ask yourself this: did you enjoy completing the side missions of the original game? If you did, then you'll enjoy Bring Down the Sky. If you thought they were an extra chore best left alone, then you'd probably be better off leaving this episode alone too.