While it might not stand up to Ontario cold, the Poncho Liner UnderQuilt is very popular.
The no-sew version is here
; scroll down to post #7 for the revised, better, version.
The sewn up version is here
I'd rate the no-sew version at about 45* F (7* C) for warmth. The sewn version doesn't gain much in warmth unless you add insulation. With a layer of InsulBright (a mylarized fleece), I'd rate one at about 32* F (0* C) for most folks--I've been as low as 25* F (-4* C)
comfortably with appropriate clothing for the temps.
Not sure about availability or costs of liners in Canada, but I get 'em for ~$20 to $30 US here, depending on quality, at the local Army/Navy surplus store. Figure ~$40 for my sewn up version, between the shock cord, cordlocks, mini 'biners, and added insulation.
An issue liner comes in at ~1.5 lbs (~0.68 kg), and adding insulation to it may bring it up to ~2 lbs (~0.9 kg). Doesn't pack down particularly well (synthetic fill never does), but it's warm and cheap.
Hope it helps!