Hey, if you want cheap there's always things like craigslist.
Tons of people are selling high-end bikes on there for less than half of what they're worth.
You can use this site:
http://www.bikepedia.com/ to get more info and see what they're really selling.
You're not gonna have the latest and greatest, but if you're not competing it doesn't matter. Good bikes last for decades and thousands of miles anyway.
If you can find bike shops that will tune up / repair / clean you bike X times a year for free if you buy new from them, it might be worth considering... but meh. By buying used you stand to save 500+ bucks anyway, I doubt they'd ever give you that much money's worth in services for free.
And I imagine you already use clipless pedals, but just in case you're not.. definitely get those. Get the shoes from a shop who can fit them and install cleats properly to your feet.
If you plan to shop around town with the same bike, then definitely get a used one you won't cry over when/ if it gets stolen. And don't buy a hybrid bike. Ever. Come on.