The Talmud (Chagiga 8a) writes that mas, the word used here for taxes, indicates a secular/political tax, rather than a religious one.
This seemingly irrelevant event may be included in Megillas Esther because, as R’ Avigdor Miller writes in Torah Nation, this verse gives honor to Persians, and could be an additional proof that the sefer was not written later, as fools claim.
The Akeidas Yitzchak adds that this is mentioned here to contrast Jewish leaders with gentile leaders. Jewish leaders focus on what is best for their constituents, whereas gentile leaders typically attempt to benefit from their charges. This is especially unjust considering the Ibn Ezra’s opinion that the taxes were even placed on nations not under Achashverosh’s control.
The Vilna Gaon notes that the gematria of mas (“tax”) (40+60=100) and the gematria of vi’iyey (“and islands”) (6+1+10+10=27) supports the Midrash that says that of Achashverosh’s 127 states (Esther 1:1), 100 were on land and 27 were islands.
The Talmud (Megilla 11a) teaches that Achachverosh felt the need to tax because the economy of Persia began to suffer. Despite the vast wealth Achashverosh displayed earlier (Esther 1:4), he lost much of it.
According to the Rokeach, this tragic loss is hinted to in the last letters of the words “es osher kivod” (“the wealth of the glory”) in that verse, which spell teired (“you will go down”).
Perhaps this can be explained by the the Targum’s opinion that Achashverosh exempted the Jews from paying taxes, and was compelled to increase the taxes of all other citizens to make up the difference.
According to Shelom Esther, Achashverosh was concerned that pro-Haman forces were still plotting rebellion. The taxes were meant to see if any group refused or delayed. That was one way to weed out any potential traitors.
Finally, R’ Moshe Dovid Valle notes that, as this verse mentions the word melech (“king”), it references H-Shem, the King of kings. He approved of Achashverosh’s taxes because He wanted the gentiles to feel how the Jews suffered.
The Jews responded to this news with a total of six actions: they mourned, fasted, cried, eulogized, and donned sack and ash. The M’nos HaLevi writes that there is significance to this number. These six actions correspond to the six days in which the Jews participated in Achashverosh’s party (see Esther 1:5). Indeed it was a seven-day party, and the Jews took a break from the last day because it was Shabbos.
Since the verse that describes Achashverosh’s party (1:4), the verse says the party lasted for many days (yamim rabim), and gives the number of days as 180, R’ Yehonason Eibshutz wonders why the phrase “yamim rabim” is not superfluous. He answers that this phrase refers to the kind of days they were, long summer days, concluding with Yom Kippur. This is the day on which no Jew sins. In fact, he adds that the gematria of the Satan (hasatan) is 364 (5+300+9+50), one less than the total amount of days in a solar year, indicating that the Evil Inclination has no hold on us for one days out of the year – Yom Kippur. Therefore, there were only six days for which the Jews needed to atone.
The Ginzei HaMelech writes that the Sfas Emes views Megillas Esther as the beginning of the Oral Law. Mordechai was even a member of the Anshei Kenesset HaGedolah (Men of the Great Assembly) that began the establishment of Rabbinic law. The Oral Law is represented by the number six, as that is the total number of Orders in the Mishnah – Seeds, Festivals, Women, Damages, Holy Items, Purity. The Jews mourned in six different ways in to show their new-found reverence for the Oral Law.
Interestingly, according to the Vilna Gaon, there are not six actions here, but five. In his understanding, the great mourning is not a separate action, but is one general action described with the remaining five detailed descriptions. According to him, these five correspond to the five actions Jews are supposed to take (Mishnah, Taanis 1:3-7) when they are suffering agriculturally.