I personally don't see much point in spending a whole lot for a commuting single speed bike, since we're likely to weigh it down anyway with accessories and the commute load, and we're not gearing it for a high top end speed. I'd draw the line at hi-ten steel and bottom end cranks though.

I've been pleased with a Dawes from bikesdirect. $200 currently. The Kilo TT is a solid alternative if you want a step up in quality.