Not all motives are created equal. Don gives Charlie a long list of suspects with the same motive. Charlie uses an asymmetric threat assessment to see which of their suspects were the most affected by their motive and most likely to act.
George is anxious to get out of his impending marriage to Susan. Kramer suggests that he ask for a prenuptial agreement, knowing it will signal Susan that George isn't committed to the idea of marriage and pushing her to back out. But in order for George's action to be credible, going without a prenup must cost him something. Susan knows this and willingly signs, saying, "Prenup? I make more money than you do! Sure, I'll sign!"