A danake/obol it seems to be
Suidas s.v. Akherousia :
"Akherousia (Acherusia) : A lake in Haides, which the dying cross over, giving to the ferryman [Kharon (Charon)] the coin which is called a danake." (emphasis mine)
Callimachus, Hecale Fragment 31 (from Etymologicum Graecum s.v. Danakes) :
"Danakes (Danaces) : a barbarous coin more than an obol, which used to be put in the mouth of the dead."
Danake also can mean the silver coin of the Persian empire, so this could be the danake of the sources.
Other choice, the Greek Obol.
Aristophanes, Frogs 180 ff (trans. O'Neill) (Greek comedy C5th to 4th B.C.) :
Herakles : An ancient mariner [Kharon (Charon)] will row you over in a wee boat, so big. The fare's two obols.
Dionysos : Fie! The power two obols have, the whole world through! How came they thither!
There are some websites that may imply any old silver coin, but I could find any sources on that.