Create a store within your preschool classroom. Arrange toy shelves to resemble store shelves, and place price tag stickers with simple prices on some of the items on the shelf. For example, a stuffed animal can have a 10 cent price tag, while a bucket of building blocks can be worth 75 cents. Arrange your classroom in this manner before children arrive for the day.
Hand each child a plastic sandwich bag "wallet" with the child's name printed on it as they arrive in the classroom for the day. The wallet should hold several coins of each value. You can use real coins or pretend money. You may also want to add pretend paper money in different denominations.
Provide adult supervision while children shop in the classroom store to help children decide how best to spend their money for the day. Explain to children they will be "purchasing" their toys to play with for the day and will receive their coins back in their wallet after the toys are returned to the shelves at the end of the day.
Have an adult act as a cashier, using a calculator to add up the children's purchases and showing them the total amount. The cashier can also help the children find the correct coins and make change.
Return items to the store shelves at the end of the school day, and reimburse children for their day's purchases. Keep the plastic sandwich bag wallets in the children's cubbies and play the game another day. Children learn best through repetition of learning games and activities.