I've had to economize on clothing as well and as already said there is no one piece for colder times of year. My winter travel is in the Northeast mountains. You can't get the best and most ultralight on the cheap but you can do alright and be light. Both systems assume synthetic or wool boxer briefs($10 wal mart synth). I've accumulated some decent layers at clearance prices over the years but I have also put thought into cheap off the shelf clothing as I work for a non-profit outdoor therapeutic program with a very limited budget. I need full side zips for skis/breathability but if you are able to do without you can save some.
Shoulder seasons:
EMS Zip offs ($30 sale)
Dri ducks ($20 for the set)
Mid weight long underwear (mine are merino and were a gift but you can get poly for $20 at Campmor)
(I am contemplating going back to light shorts and wind pants instead of the zip offs which take longer to dry but have the pockets I love, if you buy at a big box store and cut out the lining you can probably get both for $30 maybe even lighter than zip offs)
$70(reg price if you go with cheap shorts and windpants)
Winter:
Thin EMS stretch woven pants ($25 clearance as said earlier many tights will perform somewhat similarly for around $50 off the shelf or go with the windpants and a light poly base layer $40 off the shelf walmart/campmor)
EMS full side zip coated WP/B ($30 clearance or $35 regular price from campmor) for rest stops and our highly variable weather
MT Hardwear full side zip puffy pants ($60 Sierra trading post. Harder to find puffy pants cheap off the shelf campmor has fleece pants (full side zip no longer available) $30 that might be ok) camp wear/sleep system
$105(off the shelf)
$155 combined off the shelf