In reply to houghta:
Three alternatives that I can think of are the blizzard bag, which has insulation so is warmer and packs small, your standard orange bivvy bag, which is a third of the price and you don't feel so guilty bumsliding down snow on it. And finally a normal bothy bag, which would be warmer and useful on more occasions if there is more than one of you?
I have all three, I'd carry the blizzard bag over the orange bivvy as an emergency backup as it's warmer if I had to use it, but normally am out with more than me, so make sure there is a bothy bag with us, if there's a risk that needs it.