Create the calculated field in the report footer for each of the subreports and then create an unbound field for each subreport on the main form. Using the build button on the Control Source in the fields properties, reference back to the calculated field in the relevant subreport, ie [SubreportName].Report![FieldName]. Obviously you need to replace these with the correct report/field names. If you want to you can always make these fields invisible, you don't have to show them on the main report. You would then create your final total text field and sum each of these subreport totals.
Hope that helps!