If you live anywhere near an REI, be sure to watch for their "Garage Sale" events where they sell off all the items that have been returned for whatever reason. I've picked up several different bags over the years, all essentially brand new and under a hundred bux. You never know what you'll find, of course, but at the Denver store where I used to go, there always seemed to be a pretty decent selection. If you're willing to remain a bit flexible, it can be a great way to score a bargain.
There's another store in Denver called Wilderness Exchange and they do internet sales as well; they often have demos/seconds/closeouts at good prices. Worth a gander...
http://www.wildernessexchangeunlimited.com/If you sign up for their email list, you'll get a monthly 20% off coupon which can be used online too. (There's one right now that's valid through Monday.)
I tend to be a cold sleeper so I use a warmer bag than some other folks might. I don't have any idea how they come up with temperature ratings, but keep in mind that it doesn't (for me, anyway) mean you'll be toasty warm at the rated temp. I mostly use a 20 degree North Face Cat's Meow and have found it comfy down to around 35 degree nights if I layer up clothes too. If you'll be camping where it's
literally 20deg, you may want to consider a warmer bag. I'm sure everyone has different experiences; I prefer to carry a little extra weight and get a good night's sleep.