...does a converted 3-D film compare to one that was shot with a genuine 3-D camera? It's not as good, but you probably wouldn't notice anything amiss unless you'd seen a lot of 3-D flicksIt goes on to say that some directors/studios may continue to be shot in 2D and then converted to 3D because it's cheaper:
Still, some directors of 3-D movies choose to convert after the fact, rather than shooting with stereo cameras. One reason is cost. Top stereographers charge millions dollars for feature films. A full conversion from 2-D to 3-D usually costs somewhat less, although it can run into seven figures.Slate: How Do You Convert a Flat Movie Into 3-D?