My opinion?
Considerring the tech in Kinect (Lots of hi-tech stuff in Kinect whether it works fully or is seen as a glorified webcam) I'm not suprised by the price for 360 users to jump in. I reckon they'll probably break even on the current Kinect price and obviously make some profit on the games. MS/360 fans are pretty rabid so I think MS believes they'll buy it at this price point.
New users are who MS are going over to try and expand there user base by introducing a $300 360s Kinect. This they may be making a loss on (although a minor one) as they desperately try to bring the casual market the Wii has opened up. In both I think MS will do well this holiday season. The crunch factor will be whether the technical problems are true and if the consumer cares. If they're bad and annoys consumers, Kinect could fail long-term due to negative word of mouth.
Sony seem to be of the opposite opinion and look to be selling to current users as you said. I think they may be of the view that PS3 Move is too expensive to bring in casuals.
Problem with both MS and Sony is that I'm not seeing enough branching software. All of Kinet's software is casual and most of Move's lineup is core. There's nothing in-between which is what the Wii had.
