Shaun,
Does it have to be a wool sweater? If not, i would recommend one of those expedition/winter weight "baselayer" Polypropylene tops as sold per military surplus or the like. Usually they have a high, zippered collar and brushed fleece on the inside (smooth outer).
Cheap, very warm, great moisture management, and fairly light. The big weakness of woven PP in my mind is the odor buildup and retention, but it's less of a problem with mid or insulating layers (unless you're sweating it out), especially if you're wearing wool or antimicrobial baselayers.
If you combine this with a a baselayer, and a MB UL TW jacket should be more than fine at those temps. As someone that runs warm as a trend, i think i could bring down that combo to 25-27f for camp.
I have one of the PP tops i was talking about. Weighs i think 11 oz. Quite warm for the weight. Would love to try a full fleece polypropylene top, but they are very hard to find.