I have used windproof sweatpants designed for cross-country skiing, and they worked great. I think they were originally ~$200, but I found them for appx 75% off. This is probably the time of year to get a deal on them.

They give me plenty of room to move, allow me to sweat, and fit very well over my summer cycling shorts. They also fit close enough to my leg that I didn't need to wrap my ankle to keep it out of the chain.