A fork will eat into your budget a significant amount, if you can find one. I'm following forks on the bay right now, and they're all running $100+ for a decent 1-1/8" road fork. Then you'll have to get headset, seatpost, bars, pedals and other things that don't come with the group... Even if you get a good deal on each, $10-20, they all add up.

I'd say keep shopping for a complete bike, check eBay and local shops (if they do used) as well. SOMEONE will have a road bike for under $500 SOMEWHERE, even if they have to ship it to you.

Nice Vintage Cannondale Criterium 54cm Road Race Bike Turquoi Blue | eBay

Trek 2120 Carbon Fiber Road Bike Campagnolo Gruppo Great Condition | eBay

Aluminum Specialized Allez Road Bike 58" | eBay