John Gill Commentary Numbers 3:47

"thou shalt take five shekels apiece by the poll; after the shekel of the sanctuary shalt thou take them (the shekel is twenty gerahs):" — Numbers 3:47 (ASV)

You shall even take five shekels apiece the poll
Or head; every firstborn, or his parent for him, was bound to pay five shekels, which were about eleven or twelve shillings of our money, and which was afterwards settled as the price of such a redemption, (Numbers 18:16) ;

after the shekel of the sanctuary you shall take [them] ;
being full weight according to the standard there kept:

the shekel [is] twenty gerahs; (See Gill on Leviticus 27:25).