In reply to tango_kid:
I wouldn't worry too much about down getting wet, unless you frequently find your kit soaking through?
I keep mine in a lightweight dry bag (eg the Exped ones) inside my rucsac, which is fine for keeping it dry during the day. If you've got a reasonable tent, it's unlikely to get wet at night unless the weather's truly dreadful?
You perhaps need to take a bit of care to not bring dripping wet jackets into the tent and dump them ontop of your sleeping bag, but that's about it. I sometimes put a lightweight bivvy bag (Rab Survival Zone - though they seem to cost £100 now, so would take you way over budget) over the top if it's likely to get wet (eg snow hole)
But I agree, you'd only get a decent down bag by buying second hand (though you can get some absolute bargains that way - I rarely buy anything new any more, thanks to kids making me worse off than I was as a student). Alpkit would also be worth a look - at bit over budget, and not the lightest of down bags, but a reasonable all round compromise. I know less about the top end synthetics.