In reply to samparsons:
Decathlon's Wed'ze light down jacket is fairly cheap, but certainly not the best. Very lightweight shell, and quite lightly stuffed (~100g of 90/10 down, which they appear to be claiming is 660fp), giving a total weight of ~250g.
http://www.decathlon.co.uk/warm-ultralight-down-ski-jkt-id_8281847.html
Somewhere else on the price scale is Rab's Microlight Alpine down jacket, which has a much more sturdy-feeling shell, and is better stuffed, with probably better down (140g of claimed 750fp); that's why it weighs 445g and costs ~3.5 times the price...
http://rab.uk.com/products/mens-clothing/down/-microlight-alpine-jacket.htm...
For completely bonkers price, you could consider the Yeti Cut (or Proton-Ash, or whatever they've decided to call it...)
http://www.yetiworld.com/produkte/clothing/lightweight/proton-ash.html