Rashi on Leviticus 27:2/רש"י על ויקרא כ״ז:ב׳

1כי יפלא means, if he expressly states by an utterance of his mouth (cf. Rashi on Leviticus 22:21).

2בערכך 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).