You could try using CustomAggegrate server side event to achieve this functionality.

For example you would like to format UnitPrice column to have c6 format

<ISWebGrid:WebGridColumn Caption="UnitPrice" DataMember="UnitPrice" DataType="System.Decimal" Name="UnitPrice" Width="150px" AggregateFunction="Custom" FooterText="Total : " DataFormatString="c6">

Because we set the column to have an aggegrate custom we will need to implement CustomAggegrate server side event. In this event we could also have a different format string for the aggegrate column

protected void WebGrid1_CustomAggregate(object sender, ISNet.WebUI.WebGrid.CustomAggregateArgs e)
{
    if (e.Column.Name == "UnitPrice")
    {
        // obtain the rows that need to be calculated
        WebGridRowCollection rows = e.Rows;
        Decimal total = 0;

        // loop through all rows
        foreach (WebGridRow row in rows)
        {
            total += Decimal.Parse(row.Cells.GetNamedItem("UnitPrice").Value.ToString());
        }

        // pass the result back to AggregateResult.
        e.AggregateResult = total.ToString("c2");
    }
}

Thanks. This has solved my problem.

Thanks. This has solved my problem.