User blog comment:Crimson and Darkness/DxD Updates 2~/@comment-68.133.34.96-20180911215024/@comment-26709288-20180913011910

Well they're more technologically advanced than the countries on earth plus they have magic so maybe they have a digital-like currency or maybe even dont use paper money

The writer probably didn't feel the need to expand on this because the main heroine is wealthy to the point where she could turn issei's house into a mansion without even using a giant chunk of her family's money.