The 3D extras barely add anything to the cost.
Aye IIRC depending on the version of 3D used, it's basically down to if the electrics behind the set support it.
If it's like other features on the sets, most TV's inside a range will share a lot of components, with the "optional" extras only remaining so whilst it's more expensive (or they can make a big selling point of it), once the cost of adding it becomes less than keeping two versions of say the control board, they'll make it standard.
It's a little like the way things like HDMI or Divx playback and recording to USB were either "extra's", or limited to begin with, now most sets come with 2-3+ HDMI ports because the market expects it, and it doesn't cost the manufactur anything really to put 3 on the set, whilst things like Divx playback and USB recording are often done by a chip that's needed for something else.
From memory I was reading an article about new sets on show in one of the Japanese exhibitions and they mentioned that the fact the TV's could do 3D was barely mentioned compared to other selling points (low energy, connected TV etc).
So people were buying sets that could do 3D, but that wasn't as important to them as the other features.