Totaling an expression field

I have the following expression in a text box in a detail row.  I want to sum all the values in this particular expression for each customer.  How do I do this?  I tried using sum(ReportItems!TextBoxName) but I receive an error stating that aggregates for report items can only be used in report page headers or footers.  I also tried putting the expression into an aggregate function, sum(blahblah) but the report takes for ever to return data, and I know my client will not like waiting around for a week and a half for the report to run.

Any suggestions would be appreciated.

Thanks

Ralph

=iif(month(Fields!GLPOSTDT.Value) = Month(Parameters!Month.Value+"/"+"01"+"/"+Parameters!Year.Value) and year(Fields!GLPOSTDT.Value) = Parameters!Year.Value, Fields!RealDeferredAmount.Value,0)