Why does the Torah see fit to state the weight of each golden bowl (10 Shekalim) and the sum total of the 12 bowls (120 Shekalim)?
Rashi: To teach that the bowl was made of gold, but the weight was 10 silver Shekalim. 1
Targum Yonasan: Each bowl corresponded to the Aseres ha'Dibros and the combined bowls to the 120 years that Moshe lived. 2
Rashi: Because otherwise we could have interpreted Asarah Zahav to mean that the bowl was of silver, and weighed 10 golden Shekalim.
Nosei Klei Yehonason: I.e. Moshe's life was like Ketores for Yisrael. He imparted a good scent to them - Torah, Yir'ah and good Midos. (And not that the Nesi'im had Ru'ach ha'Kodesh that he will live until 120, and intended for this - PF.)
QUESTIONS ON RASHI
Rashi writes 'do not say that that the bowl was of silver, and it weighed 10 golden Shekalim.' Perhaps it was of gold, and it weighed 10 golden Shekalim!