Well, the plan is that my grandmas are going to give me gift cards for Christmas and I'll use that money to purchase the sleeping bag. I might be able to throw in a little bit more money, but there's not a lot of extra to go around (I'm a grad student, which equals no money).

It needs to fit in or on the backpack - preferably with space for an extra set of clothes. And I need to not freeze if the temperatures dip a few degrees below freezing.
What kind of price range should I be looking at if that's my requirements?