$600 per rig plus shipping. That's what it costs me to build a 1.2GH rig.
You can't buy 1.2GH/s worth of cards for $600
Maybe not new (well, not since BensOutlet.com's $140 for brand new 5870s), but anything goes on the used market.
You may be able to find the cards for $600 used, but nothing else. Regardless, I am not interested in letting them go for $600. That is half of what I paid for them and they have a lot of life left in them.