May be DataColumn.Expression Property works for you,
I'm not sure about the relation between the tables in your DataSet.

http://msdn.microsoft.com/en-us/library/system.data.datacolumn.expression(vs.71).aspx

* "PARENT/CHILD RELATION REFERENCING
*
* A parent table may be referenced in an expression by prepending the column name with Parent. For example, the Parent.Price references the parent table's column named Price.
*
* A column in a child table may be referenced in an expression by prepending the column name with Child. However, because child relationships may return multiple rows, you must include the reference to the child column in an aggregate function. For example, Sum(Child.Price) would return the sum of the column named Price in the child table."


> If I want to display Total Price as Qty * Price where Qty comes from one table and Price comes from another, what should I do with my > DataSet/TableAdapter?