Sforno on Exodus 13:2/ספורנו על שמות י״ג:ב׳

1קדש לי כל בכור, they all have to be redeemed just as other sanctified items which cannot be used for their designated purpose can be redeemed; otherwise they are forbidden for secular use. This prohibition is spelled out in Deuteronomy 15,19 in the words לא תעבוד בבכור שורך, “you are not allowed to perform work with your firstborn ox.” The amount designated here by the Torah for redeeming a human being is the minimum applicable mentioned in connection with this legislation in chapter 27 in Leviticus. Whereas a fully mature male over 20 is valued at 50 shekel, the 30 day old infant is valued at 5 shekel.