That’s a tough one, Nathalie. You will want a jacket that’s down-filled, perhaps one like the Feathered Friends Volant jacket, which has the best weight-to-warmth ratio you'll find on the market. But the Volant goes for between $300 and $350, depending on size and shell material.
I also like Mountain Hardwear’s Sub Zero Parka, which is cheaper at $280. It also looks like you can find it for $191 online in a couple of sizes. For fall 2011, Mountain Hardwear is discontinuing the Sub Zero in favor of the Hunker Down jacket, which also boasts 650-fill down and retails for $200.
Mont-Bell makes a decent down piece called the Frost Line Parka, which has 800-fill down and comes with a full hood and tough ripstop shell. That's not as much fill as the Volant, but the Frost Line Parka is still a good jacket.
In synthetic fill, Patagonia makes the very nice Nano Puff Hoody ($199), which isn't warm enough to be a standalone jacket but layers nicely under a shell in very cold or windy conditions. It has a full hood, which I think you'll find to be a useful feature.
That said, I wouldn’t rely on a single jacket to keep warm. You’ll want to think about layering, packing some good Merino wool long underwear, some fleece for a mid-weight layer, and then your insulated jacket. Layering is the only sure way to keep warm.
Good luck! I hear Mongolian sheep are tasty. Especially the boiled stomachs.