When I first tried out the Chromebook, I thought the same thing about price. But I'm pretty sure Microsoft licenses Windows for these netbooks VERY cheaply, so there's not much price to be shaved by swapping out the OS.
To hit a significant price difference Chromebooks would have to skimp on the hardware compared to their Windows/Linux counterparts. The Chromebook I have already struggles with flash, so without dropping support for that completely I don't think there's much to trim off the CPU end of things. There's a little room to skimp on the HD compared to Windows, but not a ton. So basically I don't see any route other than being ad supported, rentals or some other type of Google subsidy for Chromebooks to actually become cheaper than the low end of the netbook segment.