Everyone who crosses over to those counted must pay a half shekel, according to the sanctuary shekel, which weighs twenty gerahs. This half shekel is an offering to the LORD.
Some observe that the repetition of those words, The Lord spoke unto Moses, here and afterwards (Exo 30:17, Exo 30:22, Exo 30:34), intimates that God did not deliver these precepts to Moses in the mount, in a continued discourse, but with many intermissions, giving him time either to write what was said to him or at least to charge his memory with it.
Commenting on Exodus 30:11-16
This they shall give, everyone that passeth among them that are numbered,.... And their number, according to Jarchi, was known by what was paid; for he says the sum was taken not by heads, but everyone gave the half shekel, and by counting them the number was known, as follows: half a shekel after the shekel of the sanctuary; that is, after the standard of...
This they shall give, every one that passeth among them that are numbered, half a shekel after the shekel of the sanctuary: (a shekel [is] twenty gerahs:) an half shekel [shall be] the offering of the LORD. (h) This shekel was worth two common shekels: and the gerah about 12 pence at a rate of five shillings sterling to an ounce of silver.