Tafsiran Imamat 27:2: Rashi, Ibn Ezra, Rashbam & Rabbeinu Bahya

כי יפלא means, if he expressly states by an utterance of his mouth (cf. Rashi on Leviticus 22:21).

expresses i.e., specifies, makes clear.

כי יפלא, he will set aside. The word means something similar to the word מפרש, in the sense of spelling out details. Anyone making a vow has to clearly define what he is vowing to do.

בערכך נפשות — he expressly states that he will give to the Sanctuary the valuation attached to a life by saying, “the payment of the valuation of such and such a thing — that, however, being something upon which its life depends — is incumbent upon me” (Sifra, Bechukotai, Section 3 6; Arakhin 4a).