I scored a new MHW Phantom jacket (800 fill), very warm, for $100 on a sale (older model). It didn't have a hood so I cut one off an old jacket and used some DIY clips to attach it.
It is MUCH warmer than a fleece layer or two under a shell. Plus, when its really cold, you can wear your fleece layers, shell, AND the down jacket. For just over a pound, it's well worth it.
http://www.mountainhardwear.com/Men's-Hooded-Phantom%E2%84%A2-Jacket/OM4776,default,pd.html

Shop around, get a bargain. Doesn't have to be $500.

As far as walking around/warming up, that can be good advice. Unless it's storming. When it's cold I normally do some jumping jacks outside, or pushups/situps in the tent, then hop into my sleeping bag. Trust me, you do 50 pushups at altitude, then get in your sleeping bag, you'll be toasty.

Thanks for the all the advice gents.

I've gone for a Mountain Equipment Himal jacket. Found one for £120 ($180 ish) which is very cheap, 50% off.

http://www.mountain-equipment.co.uk/the ... ket---571/

Should be fine for Kili and Acon.

Hopefully

That looks decent. The baffled chest with stitch-through arms is clever. And a solid hood is worth its weight in gold. Good luck on the big mountains.
Don't skimp on gloves or boots.

J Cooper,

stay on the safe side. I had -13 centigrades on summit night on Kili, and considering they tell you hiking boots are okay, you may wanna be warm on the rest of your body (hiking boots are not okay for summit day on Kili if you get cold easily like me).

On Aconcagua you´ll easily be in temperatures of -20. If you wanna climb other South American mountains, again, be on the safe side. Go ahead and get a good 700/800 fill or so jacket that you´ll have for many years to come. If eventually you give up mountaineering, you can always sell it, donate it, whatever.