Three companies that I know of, and several others I would assume, have great commuter bikes that would fall in your budget. Trek's FX series has two or three models under $550, Giant has a model called the Escape (I ride one of these) that would be good, and Cannondale. Don't assume that an LBS wouldn't be able to help you with your budget - with a commute of 16-1/2 miles one-way believe me you need some quality, not a BSO. I'm using an Escape 2 from Giant. I traded an older bike straight across for it, but it retails for $420 and is great for the price.
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Trikeman