Sorry my bad it is incompatible data types, try converting to double or decimal
=SUM(IIF(Fields!col3.Value = "DEL",CDbl(0),CDbl(Fields!col2.Value)))
or
=SUM(IIF(Fields!col3.Value = "DEL",CDec(0),CDec(Fields!col2.Value)))
Far away is close at hand in the images of elsewhere.
Anon.