I understand that you need slightly more components than just an extra lens... you need movement in the angle between the lenses- just as our eyes go crossed if we try to look at our noses. Also, the auto-focusing on the two lenses must be synched. This level of control is what has kept the price of 3D cameras up. It certainly adds up to more than the sum of two cameras.
All the big action directors of note (ie the ones from the eighties) Spielberg, Lucas, Cameroon and Jackson are excited by 3D filmaking- perhaps becuase it offers a night out that can't be replicated (pirated, shared etc) in a home environment.