look on ebay. for frame only. you can usually get a decent frame for 100-400 depending on what make and model. If you are not to particular about the frame you can great deals on frames people have already tainted (painted, removed braze ons etc). But be careful when looking for vintage frames. Pay attention to condition (is there any rust) and how things are threaded (is it french)
In a nutshell, look for something in the middle of road like <a href="
http://cgi.ebay.com/Vintage-Raleigh-...QQcmdZViewItem">this</a> High end bikes hold their value, and if you are converting it you don't need all the campy bits.