The sanctity of ma'aser sheni and neta revay fruit is transferred onto a coin. The minimal value of this coin is a perutah.
The value of a perutah fluctuates and is based on the value of silver. For an explanation of the laws of ma'aser sheni and the ins and outs of this calculation, see here.
Members of Beit HaOtzar have a coin designated on their behalf. For membership, subscribe here.
For a glossary of terms related to separating terumot and ma'aserot and neta revay, click here.