Warm, light, cheap - choose two of three. If you are going to do any backpacking, I heartily recommend warm and light and skip all frivolous expenses for a while - you need a down bag. Years ago, I ponied up what was more than a month's wages for a quality down bag. It was one of the best purchases I have ever made. i would still be using that bag today if it had not been stolen.
If you are not backpacking, considerably cheaper options are available. There are some very fine synthetic fill bags that are a bit heavy and bulky, but warm enough for almost all conditions.
Geezer in Chief