The measure should be defined properly. meaning with a SQL aggregate function in the SELECT box of Designer:
SUM (sale.qty * sale.unitprice)Under properties the projection should also be set to SUM. See my other post here, please: diff between Select SUM (Table. Field) & Select Table. F
Then you will have to check for chasm/fan traps in your data model.
Andreas 
(BOB member since 2002-06-20)