My kids are a little older (10, 7 and 7) and they are really starting to understand “cost.” I do think that at these early ages it’s about helping them understand the value of money and how far it does (or does not!) go.
It’s really helpful to take them with when you have to buy a birthday present for a friend or family member so that they start to understand what all those cute, cuddly or light-uppy things cost! One of my favorite “exercises” is to talk with them about what $20 can buy: it can buy this 1 item, or these 2 items but not this other one because it’s a lot more. They start to get the idea that money is finite!