You won't find any decent quality road bikes for that low of a budget. I have the same problem. Three years ago I wanted a road bike pretty badly and I didn't have much money to spend either. So I searched Ebay. And I spent some time looking too. First and foremost, read the info about the proper bike size. It incorporates your height and your inseam. I finally found a used bike for $350. But it was only half complete. I had to continue shopping for components that were missing, shifters, stem, saddle, pedals, shoes, helmet, glasses, front derailer, rear cassette, and a repair kit to put the whole thing together. After putting it together, I paid the guy at the bike shop $100 to fit the bike. When the damn project was complete, it cost me about $900 for the complete working bike and gear. Don't forget you need good quality shorts and a short sleeve jersey. Oh, and it took me about 6 months to do it all because I would buy the components one at a time as I saved up enough $$ and when I found a decent price for a decent component. In the end, I got a pretty good bike. It's a 2005 Scattante Easton Ultralite frame originally with Alex wheels.
Anyhow, I hope this is helpful to you. If you want a decent road bike, save up some money, use credit, ask family for help, whatever. But expect to pay a minimum of $700 for an entry level road bike unless you find a local who has a used one to sell.
Good luck though. Getting on a road bike for the first time is like getting in a corvette and tapping the accelerator. You'll be surprised by the sudden response when you pedal for the first time.. and I love riding. I actually miss it if I miss too many days.
