This won't collect at the report footer because it's data for a group - it will only collect at the group footer. If you want to do a total for the report, you'll need to change the formula to:
If you want to see a summary for each group in the report footer, you'll have to use a subreport that will run from the same tables or commands with the same parameters as your main report but will just calculate the group totals.
-Dell