Multiple ways to go here. A few suggestions, in order of cost:
By far the cheapest road bikes are nice older steel frames with good period components. I picked up this ~20 year old Paramount for $300, sold the original parts with another frame, put on about $300 worth of used Campy parts, and probably had ~$400 in it. 20 lbs, beautiful shifting, rode like a dream.
Next, older "modern era" bikes; aluminum frames are cheap now as they're out of fashion. 9 speed Ultegra and DA drivetrains last forever (just replace consumables like cassettes and chains), they're light, they ride fine. My cross bike is a $100 Nashbar special frame, SRAM drivetrain from another bike, total cost about $400.
If you want to step up a bit in price, older titanium frames are as close to a "buy it for life" item as you can get in cycling. No rust or paint worries, beautiful ride, lightweight. I have about $1500 in this titanium road bike (half of that in the S&S couplers, which save $600 in luggage fees per trip!) and it's been all over the world with me. This picture was taken in a small village near Madrid, Spain.
I don't recommend piecing a bike together from components unless you already have multiple bikes; it's more profitable to go the other way (i.e., buy complete bikes and sell pieces).