How many units equals a successful DVD release?

As I grow increasingly frustrated at the failure of certain DVDs to materialize, I find myself wondering how many of a DVD have to sell for it to be considered a success? I imagine there are different answers based on single DVDs vs box sets, “deluxe” releases versus more bare-bones offerings, movies versus TV shows, etc. so as many variables as you might be able to account for, feel free.

Definitely more than 57. (Poor Kathy Griffith…)

http://www.usatoday.com/life/television/news/2004-03-24-family-guy_x.htm

According to this link, over a million is so good, it’ll resurrect your show.

Seth said they expected to sell 100,000 copies, so I’m guessing that would be the lower threshhold for most releases.