Have a discussion on why we need money. How it has originated from the early days of bartering and exchange.
Discuss how you get/ earn money (many children believe you just need a card to pay for items, or you go to a machine put your card in and cash comes out). They believe it is endless, they do not understand why we have banks or cards, or how the cash gets into the bank.
Discuss why things are different values.
If you can let your child see, feel and explore a range of real coins and notes. Explore and discuss the numbers, words, pictures on real money.
If you can get out to the shops let your child pay for items, or actively watch you.