It’s known that a voluntary fast can be redeem by donating tzedaka, and I always assumed that the equivalent of the cost of food for one day fast would suffice but I just learned in Chesed le Avraham (4:45) that the value for each fast-day is a silver coin. What´s your psak on this?
The principle source for the redemption of fasts is the Rema (334:26) concerning Shabbos transgressions. The Rema writes that although it is best to fast for atonement, one can redeem the fasts for 12 peshitin each.
Although the Magen Avraham writes that this is a changing amount, which corresponds to the minimal value of a sheep, the concensus of poskim is that the amount is unchanging, as can be seen from the Rema himself, who takes it from an early source, and does not adjust it.
The value comes to two thirds of a sela (a silver coin; this is based on Shulchan Aruch Ha-Rav; five sela’im are the amount paid for pidyon haben), which is approx. 15-20 dollars.