In 90-95% of known cases, none at all. The custom is widely attested in textual sources, but apparently not that standard in practice: most burials don't have coins.

Charon himself isn't a well attested figure. Pausanias tells us that he appeared in the lost 6th cent. BCE epic the Minyas, and appeared in a famous painting by Polygonotos (Paus. 10.28.1-3), and he appears in Aristophanes' Frogs, but all the details tend to come from later sources.

The fee in Greek-language sources is generally an obol. This is probably first attested in the Frogs (138-142), which gives the fee for two passengers as two obols -- but it's also a joke, as two obols is the entry fee for the play. An obol is also given as the price in Lucian's On mourning (10) -- again, a little problematic because it's a satirical piece.

For reference, a widely used rule of thumb is that a drachma -- six obols -- is a labourer's daily wage. So the fee is effectively one sixth of minimum wage. The buying power of ancient money never translates well, so don't put too much stock in it, but it's basically a smallish amount.

In Latin-language sources, Juvenal gives the fare as a triens (Satire 3.264-7). That's a third of an as, or 1/48 of a denarius -- and again, a denarius is sometimes reckoned as a daily wage. (A denarius was probably a bit more than a drachma, but even between Aristophanes and Juvenal, buying power doesn't translate well.) Sounds pretty minimal. But, yet again, Juvenal is a satirical writer.

The thing is, this custom is cited most often to mock it. Here's Lucian's version:

Whenever one of the family dies, first they bring an obol and put it in his mouth, to become a payment for the ferryman for the voyage -- without questioning beforehand which coinage is accepted and circulates in the underworld, and if it's the Attic obol, or the Macedonian, or the Aiginetan one that works there.

(Actually Aristophanes answers that question! He says it must be the Attic obol, on the grounds that Theseus put the coins into circulation in the underworld when he visited.)

You get the idea -- this is not a story that is taken at all seriously.

And that seems to be reflected in burial practices. Here's Ian Morris on Charon's coins, in a 1992 book:

Lucian (On Mourning 10) jokes that the dead might be better off without an obol, because then Charon would not take them and they could come back to life. Had Lucian visited a few more funerals he would have known that most burials did not include a coin for the ferryman. In the fourth century B.C. at Olynthus, only 66 of the 644 graves had a coin in them; and none of the Roman graves in the North Cemetery at Corinth, of roughly the same date as Lucian's satire, were so equipped. The use of coins also went in phases. At Blicquy in Belgic Gaul, for example, coins were six times as common in the second century A.D. as in the first. Strabo (8.6.12) says that the people of Hermione did not need coins for Charon because their town was so close to Hades that they had a short cut past him. But people at Winchester, where coins only became common after A.D. 370, had no such excuse. And some graves had more coins than others. Aristophanes (Frogs 140) has Heracles tell Dionysus that he has to pay two obols, one for his servant, before he can enter Hades; but why some people needed a whole drachma (six obols) or more is not apparent.

• Morris, Death-ritual and Social Structure in Classical Antiquity (Cambridge, 1992), p. 106