"Plenty of coin" (from the dictionary example) is a common phrase. But the line from Ready Player One is unconventional. To me, it sounds like the writers are trying to make it sound futuristic/digital. He is referring to his currency within the Oasis (virtual reality metaverse). So calling his money "coin" sounds like he's referring to the currency within the Oasis, not his government backed currency used outside of the virtual world. You can also see this singular usage of "coin" with "bitcoin", which is both singular and plural.
Not a teacher
"Plenty of coin" (from the dictionary example) is a common phrase. But the line from Ready Player One is unconventional. To me, it sounds like the writers are trying to make it sound futuristic/digital. He is referring to his currency within the Oasis (virtual reality metaverse). So calling his money "coin" sounds like he's referring to the currency within the Oasis, not his government backed currency used outside of the virtual world. You can also see this singular usage of "coin" with "bitcoin" which is both singular and plural.