I have a Tennier MSS (Modular Sleep System) which is a series of bags that are good from -10º to 50º. I should mention that the coldest temperatures that I've gotten a chance to test it in has been the low twenties to high teens, and it performed well. I had to take off my base layer because I started sweating. The highest temperature has been around 65º and at that point I left it unzipped and wide open, essentially using it as an additional layer of padding over my Thermarest.

I would like to have gotten a Slumberjack SVCSS (Slumberjack Variable Component Sleep System), which is rated -30º to 30º. However, it costs about $500.00 to $700.00 depending on whether you buy the whole system together or in pieces. That, and they were only making the MSS at the time, or they changed the name to seperate themselves from other companies.

In any case, the Tennier MSS cost me between $300.00 and $350.00 two years ago, if I remember correctly.