My Daddy used to play a variation of this in restaurants when we went on family vacations. It's great for teaching math skills and making kids aware of the cost of living.
As you enter the store the players pick a prize like a candy bar or magazine. Maybe even an article of clothing if the game sponsor is flush with funds that day and doesn't want sugared-up kids in the back seat. The main thing is that the players must agree on the prize. The game sponsor puts the prize in the cart and the game begins. As you shop the players keep track of purchases. At checkout time the player who comes closest to knowing how much the basket of groceries cost wins the prize.
When I introduced the game idea to the girls their first question was could they take paper and pencils into the store. We decided that would be permissible. Then they wanted to know if they needed to account for tax. That was followed by a study of our latest Walmart bill and a discussion of what was taxable and what wasn’t. Given that I couldn’t fully understand the whole taxable thing myself we decided to keep the game easier at first by not having to account for tax.
Maybe if we progress to Level Two competition we'll include tax.