Ok, so I was looking for basically the same thing. 0 degree, lightweight and cheap. And the previous entries are right, you can get 2 of the 3, but not really all 3.
So my choice was between the Kelty Cosmic 0 degree down and the Cabela's XPG Escapist down bag. The cabela's bag is only a 15 degree bag, not a 0 degree, but it looked like it had more down fill weight-wise, and was 600 fill, not kelty's 550, so it might be equal or warmer than the kelty? The XPG is also downproof and water-resistant with a 40-denier ripstop nylon shell. Some of the Kelty reviews I read on the Cosmic stated that the down did poke through the lining, and it was not water-resistant. And the XPG is under $100 on clearance (2010 model). The only downside for me was that the XPG bag came in at 4 lbs.
Not sure if 4lbs for a 15 degree bag is a good or bad thing, but the current 20 degree synthetic bag I have is only 3lbs, so I'm going to stick with that and test it out this winter. If it doesn't keep me warm, then I'll find something to replace it with at that point.
I'm not saying go with either bag, I'm just curious if some experieced winter camper on this forum could look at the Cabela bag and give their opinion? http://www.cabelas.com/product/Cabel...h-All+Products