If you've got some real estate in your pack, it might be worth looking at a poncho liner underquilt (PLUQ). There's directions for a no-sew version here (scroll down to post #7 for the updated directions) and a sewn up version here. Both will get you down into the mid-40s, no problem. If you add insulation to the liner (I did), it can get you much lower. With a single layer of InsulBright, I was able to take one down to 25* comfortably and 21* without freezing to death (I was cold that night).
Total cost on a no-sew version (no insulation added) will run you anywhere from ~$30 to ~$50, depending on how much your local Army/Navy store charges for a poncho liner. It'll weigh a little less than two lbs, but is very bulky.
Hope it helps!