I took the advice of others on here, and saved up for a western mountaineer badger sleeping bag. it didn't get down to single digits, but temps got down to the teens in a two man tent up on the mountain, and I am SO glad I had that bag. its lightweight, slightly oversized for a mummy bag (which is good for me because I don't like them....LOL), and was worth every penny!
I assume your backpacking when you mention a backcountry trip. If you are truck camping, you can definitely get by with cheaper options by simply nesting one bag inside another, but for a backpack style trip, those lightweight warm bags come with a big price tag.
not only do you want a good bag, but the pad is just as important. The sleep system is an area that (IMO) you don't want to try and cut corners or save $