It largely depends on expectations for whether it is a flop or not, but I think the break even for PS3 & 360 is likely around 300-400k on the average. Obviously some titles will be less but some will also be much more.
As for the Wii I would probably guess around 180-240k or so for the average.
To be clear this is speculation based mostly on developer reactions to game sales and narrowing it down based on that.