Pidyon Nefesh is a very special and powerful prayer and blessing that has the spiritual strength of a sacrifice in the Bet Hamikdash to atone for sin and to mitigate severe judgments. The bigger and more pious the tzaddik, the more effective the "pidyon", which literally means "redemption", where one's money is literally a sacrifice for one's body and soul. The pidyon money "redeems" the nefesh, or soul, in other words, "Pidyon Nefesh." This is used when a major salvation is needed, such as in the case of a dangerous illness, a severe court case, or as a rescue from trouble or danger. Rebbe Nachman of Breslev says that Pidyon Nefesh is the "sweetening", or mitigation of harsh decrees and serves as a salvation from any trouble, for the main cure and solution of sickness is the pidyon (see Likutei Moharan II:3).
Any amount is acceptable with a miniumum of 26NIS or $7 or equivalent, but for a Pidyon to be especially effective, one should give with dedication. One cannot hope for a major salvation without giving to the limits of one's capability.