I will put 2 numbers, the first is the answer to the opening (To send a message), the second to be a succes in economic terms (to the developer/publisher)
MadWorld : 800k : 400k
Conduit : 2 mill : 600k
But both games sells have to count with one condition they have to reach that number FAST!!! In 2 or 3 months, these are the kind of game where "some" legs don't work for the publisher, yes they will continue to make money for a long time, but they will not send the message.
I think the Conduit can do it, but madworld won't for a lot of reasons (graphics style, rating, genre, etc)