I bought both of my 5830's at $99 a piece. I can get 305-307mh/s steady. That works to 3mh/1$ already.
True but you also need a system to run the card.
I proposed cheap old computers from craigslist ($20-$30) with PCI-e slots as a solution, which would bring such a rig very close to the desired $3/MH ratio.
The computer I linked doesn't run PCI-e but you get the point, this kind of stuff is really available out there for very cheap.
Some people might even throw these away rather than sell them.
At current prices of components, this isn't doable with any other card than a 5830 (if you're lucky enough to find one)
Also not counting the insane deals Bitjet got above, they are very rare or one-of-a-kind.