The Kelty Cosmic Down 20 seems to be most recommended for those worrying about budget. Its current retail price is $170.

It's recommended by highly experienced backpackers including Paul Magnati, who has backpacked in Colorado (and other Rocky Mountain states) for a number of years.

Please don't associate "fill power" with the temperature rating! The higher fill power only reflects the warmth per unit of weight. Down fill power. In this article the author compares the Kelty Cosmic with the Western Mountaineering Ultralite. The primary difference is the 12 ounces heavier weight for the Kelty (otherwise the two bags have similar features). It might also make a few years' difference in its useful life, assuming heavy use. Only you can decide whether the $300 price difference for the WM is worth it. Note that the current model Kelty Cosmic uses 600 fill Dri-Down.

Do note that even with Dri-Down, you need to make extra effort to protect down from moisture, although my bitter experience is that a soggy synthetic bag is no warmer than a soggy down bag!

If you haven't already read it, here's an outstanding tutorial for beginning backpackers: For beginners, by Andrew Skurka.