Very strange, I have just created a query with Quantity, UnitPrice and ExtendedPrice as a calculated field, and it shows ExtendedPrice as currency with no type casting.
SELECT TblExtendedPrice.Quantity, TblExtendedPrice.UnitPrice, [quantity]*[unitprice] AS ExtendedPrice
FROM TblExtendedPrice;
In the table I have Quantity as data type Number, Long Integer and UnitPrice as data type Currency? Shouldn't need to change data types?