I was going to recommend the Campmor bag as a budget down bag. If you're willing to try something different, and can sew, I suggest checking out the thru-hiker.com quilt. Right now they have a kit that uses superlight Momentum 50 fabric. Since you make it yourself, you can add extra down to the baffles where you're coldest. Add a TiGoat bivy and you'll have a flexible lightweight sleeping kit. If you put together this kit, I suggest doing the footbox differently. I like it to have a zipper and drawcord so that you can completely open it up like a blanket for warm nights.