Following up the question raised in yesterday’s post, the Chazon Ish explains that although a person is usually required to spend no more than 1/5 of his wealth on any mitzvah, the mitzvah of pidyon haben is different. The financial obligation to buy an esrog or talis or other mitzvah item is itself not a mitzvah – it is just a means to an end. In these cases the Torah places a cap on the amount one is required to expend. However, the obligation to pay a kohen for pidyon haben is not just a means to an end, but is the mitzvah itself. Since in the world of dinei mamonos there are no limits on the obligation to pay back debts, e.g. one cannot tell a lender that the terms of payment exceed 1/5 of one's income and therefore one is exempt from payment, there are also no limits on the obligation of payment for pidyon haben.
By defining pidyon haben as working within the parameters of dinei mamonos, the Chazon Ish opens a pandora’s box of other issues, some of which we dicussed in the past (link)– e.g. can a kohen be mochel payment of pidyon haben money? If we treat the mitzvah purely as a financial obligation, the answer would seem to be yes, but if it is a mitzvah like other mitzvos, than not. (Daf Yomi learners: see Tosfos 30b d”h zar regarding whether a kohen can be mochel tashlumei terumah).