zip-off pants make a nice wind-blocker and don't have to cost too much:
http://tinyurl.com/3ynb7y3

I, too am a multi-moder in DC area. I also like pockets for my Metro card, etc. I do sometimes wear longjohn's under my non-bike shorts anyway, but usually just the thin zip-off hiker-style pants are enough to keep me warm until it gets into the 30's.