Each one who is numbered in the census shall give this: half a shekel according to the shekel of the sanctuary (the shekel is twenty gerahs), half a shekel as 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.