I have been very comfortable in the low 20°s with a base layer of Target wicking thermals ($9) and a pair of wind/waterproof running pants ($10). I find I need to wear some sort of brief/trunks under the thermals because of the 'texture' of the weave. If it is in the 30°s, I replace the wind pants with knee warmers (Nashbar $15) over the thermals, with some running shorts on top.
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"Bad facts make bad laws." FZ