Seemingly, Esther’s point is that the loaves of silver paid during Haman’s deal with the king (Esther 3:9) was a bad deal for the king. However, as the Maharal points out, Achashverosh returned the money (Esther 3:11), so an alternative interpretation is necessary.
According to Rebbetzin Heller, Esther was saying that the humiliation that the Jews would experience would not justify bothering our great king, Achashverosh; it would be beneath his dignity to do such a thing.
Also, the enemy – Haman – is not considering the loss to the king because he only cares about himself.
As the Talmud (Megillah 16a) interprets this phrase, Haman does not care about Achashverosh. First, Haman advised the killing of Achashverosh’s beloved Vashti, and now Haman has set his sights on the king’s new beloved, Esther.
The Ibn Ezra adds that Esther was saying that Haman cares so little for Achashverosh, that he does not even mind Achashverosh’s loss of tax revenue in killing out so many citizens of the realm.
According to Rashi, Esther is pointing out that if he had cared about Achashverosh, Haman would have advised him to sell the Jews and keep the money.
R’ Yehonason Eibshutz says Esther was protecting Achashverosh from an assassination plot; if he will kill her, then he would kill the king, as well.
Like the Rokeach, the Yosef Lekach writes that Esther’s point was that enslaving the Jews is permissible by the Torah, but trying to kill them off is against Torah. Therefore, Achashverosh risked being punished for this, and Haman would not care if he were.
The Dena Pishra writes that Esther’s point was that, as a king, Achashverosh could uniquely appreciate what a loss the Jews would be to H-Shem, their King, and how He will respond for the sake of His subjects.
According to the Alshich, another point Esther was making is that, in returning the silver (Esther 3:11), Achashverosh essentially sold his own wife as slave for free.
The Holy Shelah interprets “the king’s damage” as pain being inflicted upon the King of the World.
The Ketones Or quotes the Talmud (Taanis 3b) that it is impossible for the world to exist without Jews. Accordingly, Esther’s point was that Haman does not care about that, so this plot is not to Achashverosh’s benefit.