Since I don't know what prices bikes are in the US I won't talk about anything new, but here's my experiences with used bikes. About Feb 2003 I bought a used Giant for AUD800, prob 500 USD - it was wondeful, in pretty good shape and I had a friend check it out. Jan 2004 the bottom sloping tube broke all the way through, adn yeah I'd done 10000+km on it, but there's no way of knowing if the problem (weakness in the frame) was there when i bought it or if I caused it. You can't tell if a used bike has got a weakness in the frame, so I recommend against used bikes, & if you do buy one get it checked out by a bike mechanic you trust first. At least the new bikes come with warranties (often lifetime) on the frame.