Groupset and components are going to be your biggest cost.
You should be able to pick up a decent used aluminum frameset for around $400.
A basic wheelset will go for about $200, add another $100 for tires and tubes.
Bars, stem and post can be about $100-$150 for used, aluminum stuff.
Saddle, you want to get something decent. So, plan to spend $100-$150 there.
A groupset will probably cost you about $400.
Then figure pedals, cages... $200