I'm guessing you are seeing the division by zero error type error here?
You may be able to modify something like this to get what you are after. Swap out the 0 for is nothing.
=Iif(Sum(Fields!EXT_SALE_AMT.Value) = "0.00", "-100", Sum(Fields!MARGIN_DOLLARS.Value) / Iif(Sum(Fields!EXT_SALE_AMT.Value) = "0.00", "1", <<<<<<<<<<< If the value is 0 this makes it 1.
Sum(Fields!EXT_SALE_AMT.Value)))