I had read previously either on this site or another of the RE blogs that the builders cost is typically $83/sq foot. Perhaps, with higher end features, it is $115/sq ft.
If that is accurate, then assume that an average 3,000 sq foot new construction home sells for $1 million. At this sell price, there gross sell is at $333/sq ft.
So assuming the above, their margin is quite fat and much greater than 30 or 35%. Someone can validate that. That is what I remeber reading.