It's doable, but obviously you are going to have to make some compromises in terms of weight, quality etc. The sources Andrew talks about are a good place to start. Being in Trenton, you can also consider the spring flea market in Trexlertown (www.lvvelo.org), there are usually some good deals there. And bikes direct has models starting under $500. But like Andrew said it's up to what you are comfortable with mechanically and what the primary use will be.
are you mechanically inclined? if so, ebay and bike swaps and craigslist are the most reliable places for good deals. probably bike swaps being and craigslist being better sources of bargains that ebay. if you don't want used, there are a # of places online that sell direct to consumer frames or bikes... what are you looking for? a race bike? ss? do it all?
There was a guy at the race last weekend on a Motobecane Single Speed that seemed like a good deal. He only paid something like $400 on bikesdirect.com I believe. Get a single speed and you don't have to worry about the components!