I bought some cheap snow/ski pants at Dick's the other day for $20, and they worked like a charm in today's 40° weather. These aren't as bulky as most snow/ski pants, but they are insulated and they kept my legs (and my boys) warm.
I put some straps around my ankles, and then neoprene booties over my shoes, which kept the pant legs from interfering with the cranks. My feet still got a little chilly, so at the half way point I put a pair of those chemical toe warmers in my shoes. Those helped take the bite off. I was a bit warm on top though, I wore a cheap fleece jersey I got from Performance for like $12, and then my winter jacket. I could've just gone with the jacket or the fleece and a light windbreaker. Still, it's better than being cold.
If you're interested, here's what my full set-up was:
Long sleeve skinsuit as a base layer, with chamois.
Assos Airblock bib tights.
SkiGear snow/ski pants (bibs).
Long sleeve fleece jersey.
Columbia Sportswear jacket (came as part of my winter coat).
Smartwool socks and neoprene booties over the shoes.
Now for all you "I ride in 30° weather with shorts and a long sleeve jersey" types, bug off. We don't all have the same comfort level or metabolism. I've been looking for something to keep my legs warm for a long time, and it looks like I've finally succeeded.
So even though I was nice and comfortable for a change, it's been like 3 weeks since I've been able to ride, so I totally sucked. I didn't even make 12mph.
Oh well, maybe now I'll be able to get out a bit more often.