Look at the Kelty Light Year 20º down bag. You should be able to find one online that squeaks into your budget. I have one, and so does another member's wife, and we both like them a lot. I think I paid around $130 for mine a couple years ago, and someone else here said they found one for around $80 not that long ago.

Sound like a great trip you have planned. Let us know how it went, and we all love to see pictures!!!