If I were you I wouldn't look for a brand explicitly... I'd follow craigslist for bikes that are a good deal.

I got my Trek commuter for $120 (originally $550). People have a tendency to buy things like bikes thinking that they'll use them more than they do (Like those people that buy a guitar so they'll learn to play), a year or two later they end up just selling them to get more space.

Just a thought.

Curious what city you're commuting in though.