http://seattletimes.nwsource.com/htm..._down_gau.html
According to that link, Natal will cost around $200 on top of the 360 console cost. Unless it turns out spectacular, that price could easily ruin Natal.
If this is true, it seems to me that Microsoft is trying to get Natal out there too soon before it's economically feasible just to try and cut into the casual market.