I just thought of this: my kids have loved playing store with this type of toy cash register that includes a real calculator in it.

http://www.diapers.com/Product/ProductDetail.aspx?productid=22751

It came with some fake cash and a credit card. You can talk about how buying two of something costs twice as much, what price is reasonable, what if it's on sale, coupons/subtraction, all kinds of money ideas, during pretend play. We got a lot of play mileage and a lot of math mileage out of it.

DeeDee