Sleeping bags are a crazy adventure all in themselves. Some manufacturers seem to outright lie about temp ratings while others are spot on. My advice is to learn as much as possible about bags. Visit all the websites. Then looks for deals. Be sure to visit Feathered Friends and Western Mountaineering. Other 'major' name brands make good bags too, but FF and WM are often considered supreme. Also consider the debate between down vs. synthetic fill.

I purchased a Western Mountaineering Ultralite bag 2 years ago. I wanted a bag with continous baffles, a full zip, down, and fit my 5'5" frame well. I wanted the full zip and continuous baffles for the most versatility... I can rearrange the down for more/less warmth and use it like a quilt if I want.

For $100-$200 you can certainly get a decent bag. Do your research on customer reviews. Watch for clearance sales. Another option... make yourself a "Ray Jardine" quilt for around $130-$150.
I always forget and make it more complicated than it needs to's just walking.