The Kelty 20*F Cosmic Down sleeping bag has had good reviews and can be found for $100 or less. It's quite a bit heavier and won't last as long as a top quality down bag (example: Western Mountaineering or Feathered Friends), but it's sure a lot cheaper. It's also lighter and, properly cared for, should last longer than a cheap synthetic bag. The newer version is EN13537 rated so you can assume the temperature rating is fairly accurate, unlike most inexpensive bags where you need to assume it's 10* less warm than the fictitious manufacturer rating.

It would be a good bag to start out with and should last long enough for you to save up for one of the much lighter top quality bags, assuming you become hooked on backpacking!

I can't stress enough the importance of a good, well-insulated sleeping pad!