Cheapest Luger I've bought was $750 about 4 years ago, a byf 41 almost all matching except for the wrong sideplate (correct numbers stamped outside but the inside numbers didn't match my gun).
Here's a decent little mismatched shooter for under $600. I thought these deals were dead and gone unless you found a little old lady trying to get the nasty gun out of the house. The only things I've seen under $600 were the "basket case" variety.
http://www.gunsamerica.com/guns/976699314.htm
I realize this isn't one of those glamorous posts about the latest one-of-a-kind Krieghoff, but bottom feeders need love too!