A thought based on the Taxi Driver trailer
Descending to the Underworld is a tradition for Greek heroes. They went for such stellar reasons as 'It's a convenient place to stop and ask directions' and 'Well, everyone else is doing it'. Hercules's most famous Underworld-related exploit was the capture of the hound Cerberus - and Sam's already done the hellhound thing. But it is also true that he had to go down and rescue fellow hero Theseus, who got trapped down there with his friend Pirithous due to agreeing to the most stupid dare in human history. Hercules was able to bring Theseus back, but Pirithous had offended the gods too much - and so had to stay in the land of the dead.
Also Taxi Driver, as well as referencing the Robert De Niro film, would presumably recall Charon the ferryman, who took souls across the river Styx (or sometimes the Acheron), and thus presumably Sam's 'fare' down to Hades (traditionally one obol - a silver coin - placed in the mouth of the dead, but since even Aristophanes made jokes about inflation, who knows what Sam will have to pay). Of course it would be a car, not a boat: this is Supernatural, so there's always a road. But if they cross a bridge at any point I may die of glee. :)