I highly recommend the Planet Bike KOKO rear rack. It only costs about $35-50, available in black or silver, and works as well as racks that cost 3X as much. Very adjustable, which is a key feature during installation.

You might be able to get by with a rack-top bag rather than panniers. The trick is to leave heavier, bulkier items at work -- such as shoes, bath towel, bath supplies. I commute year-round, 30+ miles round trip, and I can can fit all the gear I need to carry in a rack-top bag. Some good inexpensive options are made by Banjo Brothers, Louis Garneau and others.