Have you ever looped through the entire dataset to get the totals you need to put in the footer? The DataTable.Compute method eliminates this hassle by retrieving commonly performed arithemetic operations like sum, avg,, etc.
int iTotal = table.Compute("Sum(amount)", String.Empty);
The preceding code computes the sum of the amount column. You can even filter using the second parameter:
int iTotal = table.Compute("Sum(amount)", "amount > 5" );
Note: You will still need to loop through the items in the ItemDataBound event to assign it to the footer.