I have a 13 mile commute, and I'm looking to buy a new bike I'm currently riding a mountain bike from Wal-Mart, takes me about an hour. When I get my tax return, I plan buying a more appropriate bike for the commute. I'd like to stay around $300-400, and this would be the second bike I would buy, so I'd appreciate any suggestions and advice.
Used. You can find some decent used bikes in that range.  A hybrid might be a good choice.  You find people that paid $800 - $900 for bike they never used and it will sell used as low as $300.  Be patient and low ball.  Learn how to inspect a bike.  If you are on a budget you need to learn how to maintain a bike.