Western Mountaineering is one company that does appear to rate its bags accurately, for comfort rather than survival. The 20* F Ultralight Super really does take me down to 20* in just a base layer and (when below 30* F) a non-breathable rain suit as a vapor barrier. And I'm a really cold sleeper! If you can either save the bucks or buy one used, it will be well worth your while!

As a cheaper measure, REI's Outlet is still selling their Sub-Kilo bags (middle-quality down) for $160 for the regular length and $170 for the long. These are, supposedly 750-fill down. I picked up one for my 9-going-on-10 year old grandson. Watch for another 20% off coupon on Outlet stuff, which would let you get it for $130. Not as good, and probably closer to 30* F than the advertised 20*, but should last a while with care.