The basic rule with bags is "cheap, light, warm - pick two." Forget the rating, unless you are a polar bear, I guarantee it is total bunk. For example, two pound down bags good to 20F from Western Mountaineering or Feathered Friends cost around $400.

I have a down bag rated a -5C (+23F) which I have used in Yosemite in winter with an overbag and two pads down to around +15F. I now have another bag rated to -10F from Marmot which will be warmer. For three seasons, a 20F bag should be fine, if it is an accurately rated bag like mine.

If you can, buy a decent bag. Take care it and it will last a long time. Some companies like Marmot will refill down bags for a modest price if the down gets compacted, which it may do with heavy use. I bought my last bag off of Craigslist. Some people don't like the idea of buying a used bag, but mine was like new and I sent it out to be washed, so once I did that, it looks like new. I paid half retail for it. I bought a winter parka off of eBay (the one in my picture) same deal-just over half retail (which was close to $500 new).