I have seen used Specialized Dolce road bike for around $500 to $600 on Craigslist. Just remember to pay to have that bike checked out at your LBS (Local Bike Shop) before you buy.

But, I have not seen any good new road bike with drop bars for the $500 price.
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2012 Specialized Amira Elite, upgraded carbon handle bars, Jett saddle 143mm switched to 145mm 2012 Selle Italia Max SLR Gel Flow saddle

2011 Specialized Ariel Sport,suspension post,Serfas Rx Women's Microfiber saddle