What is the currency used in Percy Jackson and the Lightning Thief?

Answer 1

The main denomination is the castar (Gondor), kastu (Rohan), kast (Shire), and known as a mirian by the elves (probably derived from the Sindarin mir or "jewel"). The dwarves may have used mithril to make their coins.

Answer 2

The Olympians use drachmas, big golden coins.

Percy and his other demigod friends have drachmas on hand if they have to deal with Olympians or anyone that knows about the gods (ex. sending an Iris message)

They use normal cash for other things.

Answer 3

Wizards have their own currency, called galleons, sickles and knuts. There are seventeen sickles to a galleon and twenty-nine knuts to a sickle.