Don't do it. I just went into the city for the first time since my lease was up last may and FUCK! THEY'RE EVERYWHERE! Gangster hip hop philippinos with color matching 59 fifties. Pale white chicks with the messenger stylee. I feel sorry for kids who can't pass up the 25 mph city traffic, especially when every fucking bus driver is trying to box you right into a gutter.
But since they're mainstream now, you can get a pretty sick looker for $500 with no assembly required at any bianchi or giant retailer. If you want a cheap second hand one, wait a couple years when the fad dies. Craigslist will be flooded with em.
That's my two cents. But all my city folks love their fixies, so to each his own.