Go to walmart! Prices are estimates.
Synthetic boxer briefs (or whatever) $8 ( I wear these 365 days now)
wool or synthetic thermal underwear $15
Wind pants $15 (workout pants. mine are surprisingly VERY wind/water resistant)
Fleece pants $15

After that, you could get goretex shell pants, but probably won't need them unless you plan to sit on the snow. if so, check the ski section of a sporting goods store.
The wind pants are cheap enough that you don't have to worry much about durability, but then they are pretty durable anyway.

Oh, and if you can't find fleece pants, look for fleece pajama pants. same thing.