Laudator Temporis Acti

The earliest references to a fee paid to the ferryman Charon for passage to the underworld occur in Aristophanes' Frogs, here in Jeffrey Henderson's translation and with his notes.

137-142:

HERACLES
First you’ll come to a vast lake, quite bottomless.

DIONYSUS
Then how will I cross it?

HERACLES
An ancient mariner will ferry you across in a little boat no bigger than this, for a fare of two obols.¹⁸

DIONYSUS
Wow, what power those two obols have everywhere! How did they make their way down there?

HERACLES
Theseus brought them.<sup>19

18</sup> The traditional fare was one obol; here “two obols” probably refers to the two-obol dole introduced by the politician Cleophon in 410 (Aristotle, Constitution 28.3; IG i³ 375-77), perhaps to the price of a theater ticket (but then "everywhere is hard to explain).

¹⁹ The journey of Theseus, an Athenian hero, to the underworld was well known, and was dramatized in Critias' tragedy Perithous (date unknown, and alternatively attributed in antiquity to Euripides).

ΗΡΑΚΛΗΣ
εὐθὺς γὰρ ἐπὶ λίμνην μεγάλην ἥξεις πάνυ
ἄβυσσον.

ΔΙΟΝΥΣΟΣ
                  εἶτα πῶς γε περαιωθήσομαι;

ΗΡΑΚΛΗΣ
ἐν πλοιαρίῳ τυννουτῳί σ᾽ ἀνὴρ γέρων
ναύτης διάξει δύ᾽ ὀβολὼ μισθὸν λαβών.

ΔΙΟΝΥΣΟΣ
φεῦ.
ὡς μέγα δύνασθον πανταχοῦ τὼ δύ᾽ ὀβολώ.
πῶς ἠλθέτην κἀκεῖσε;

ΗΡΑΚΛΗΣ
                                        Θησεὺς ἤγαγεν.

270:

CHARON.
Off you go. Pay your fare!

DIONYSUS
Here's your two obols.

ΧΑΡΩΝ
ἔκβαιν᾽, ἀπόδος τὸν ναῦλον.

ΔΙΟΝΥΣΟΣ
                                                   ἔχε δὴ τὠβολώ.

The Wikipedia article on Charon's obol is better than most Wikipedia articles, but doesn't mention some recent scholarly works. In the following selective bibliography, works not cited in the Wikipedia article are marked with an asterisk:

* Caronte: un obolo per l'aldilà (Napoli: Macchiaioli, 1995 = La parola del passato: rivista di studi classici, no. 50, fasc. 282-285, pp. 162-541), table of contents here

* Francesca Ceci, "La deposizione nella tomba: continuità di un rito tra paganesimo e cristianesimo," Histria Antiqua 13 (2005) 407-416

* Francesca Ceci,"L'interpretazione di monete e chiodi in contesti funerari: esempi dal suburbio romano," in Michael Heinzelmann et al., edd., Römischer Bestattungsbrauch und Beigabensitten in Rom, Norditalien und den Nordwestprovinzen von der Späten Republik bis in die Kaiserzeit / Culto dei morti e costumi funerari romani, Roma, Italia settentrionale e province nord-occidentali dalla tarda repubblica all'età imperiale. Internationales Kolloquium, Rom 1.-3. April 1998 (Wiesbaden: Dr. Ludwig Reichert Verlag, 2001), pp. 87-95

* Jean-Marc Doyen, "The 'Charon's Obol': Some Methodological Reflexions," Journal of Archaeological Numismatics 2 (2012) I-XVIII

Keld Grinder-Hansen, "Charon's Fee in Ancient Greece? Some Remarks on a Well-Known Death Rite," in Recent Danish Research in Classical Archaeology: Tradition and Renewal (Copenhagen: Museum Tusculanum Press, 1991 = Acta Hyperborea, 3), pp. 207-218

Susan T. Stevens, "Charon's Obol and Other Coins in Ancient Funerary Practice," Phoenix 45 (1991) 215-229

* Trouvailles monétaires de tombes: actes du deuxième colloque international du Groupe Suisse pour l'Étude des Trouvailles Monétaires, Neuchâtel, 3-4 mars 1995 / Fundmünzen aus Gräbern: Sitzungsbericht des zweiten internationalen Kolloquiums der Schweizerischen Arbeitsgemeinschaft für Fundmünzen, Neuenburg, 3.-4. März 1995, edd. Olivier F. Dubuis et al. (Lausanne: Éditions du Zèbre, 1999 = Études de numismatique et d'histoire monétaire / Untersuchungen zu Numismatik und Geldgeschichte, 2), table of contents here, includes G. Thüry, "Charon und die Funktion der Münzen in Gräbern," pp. 17-30