I'm not sure if the syntax is quite right, but something along these lines might work:

=IIF( (Sum(Fields!DBA_Estimate.Value, "dsStatistics")-(Sum(Fields!DBA_Actual.Value, "dsStatistics")))/60 < 0, "Actual time is more than the estimated time", (Sum(Fields!DBA_Estimate.Value, "dsStatistics")-(Sum(Fields!DBA_Actual.Value, "dsStatistics")))/60 )

Doug