I have a campout coming up next month that might involve colder weather, and have been looking at the Modular Sleep Systems used in the military. For those of you not familiar with them, this is a four piece set that includes a light-weight sleeping bag (35 degrees +), a heavier bag (5-35 degrees), an external "bivy" cover that is water/wind resistant, and a stuff bag to hold them all. The heavy bag fits inside the light bag and then both (or just one of them, if you prefer) can go inside the bivy bag to keep out the wind and rain. Basically a layered sleeping bag system.

I've found a source that has 7 of them that are lightly used, but in very good shape for $50 each. I'm going to get down there either this weekend or next to pick one up, and if anyone else is interested in buying, shoot me a PM and we'll work something out to get one to you.

These are not lightweight bags, but at that price, I think they would be perfect for winter camping on the bike. These sell for about $200 new, and normally about $100-$120 for lightly used (military surplus).

Here's a video that shows them in action (minus the stuff sack)