If you're riding during normal commute hours, you need to be prepared for dark, windy, cold, and potentially very wet conditions. Wind chill temperatures can be below freezing. I recommend rain pants and a water resistant jacket. How expensive you want to go depends on how many hours you will be riding. Cheap stuff is not as comfortable, but that's not a big deal for shorter rides. Also, closer fitting stuff avoids the parachute effect if you're riding at higher speeds.

If you're riding in traffic in the dark, get brighter colors as well as lots of lights.