Used you can probably find something decent but anything under 20 lbs will run you more than 1k.
Last year for $140, I got what I thought was a good deal on a K2 Mach2 road bike in very good condition, been riding the heck out of it.
Previous to that I bought an used Specialized Allez that was about 2 years old for $400.
Both these bikes were in the $700 to $800 range when new.
Look in Craigslist but don't expect anything even used in the under 20 lbs range in your budget.