Parashas Vayeishev · Passuk 37:28
And there passed by Midyanite merchants; and they drew and lifted up Yosef out of the pit, and sold Yosef to the Yishmaelim for twenty pieces of silver. And they brought Yosef into Egypt.
Torah Temimah points to Yerushalmi Shekalim 2:3 (and parallel teachings linking the twenty silver to the five shekels of pidyon haben).
Sefaria — Yerushalmi Shekalim 2:3
English: Rabbi Shimon ben Lakish said: Why did the Torah command five shekels for the redemption of a firstborn? Because they sold Rachel’s firstborn for twenty pieces of silver; therefore each person should redeem his firstborn son with twenty pieces of silver (five shekels).
Derash on the verse (Yerushalmi / midrashic sugya) — not a formal named middah of the 13. The historical sale price becomes the normative amount for the redemption of the firstborn.
The five shekels of pidyon haben correspond to the twenty pieces of silver for which Yosef (Rachel’s firstborn) was sold. Each firstborn is therefore redeemed at that same measure.