public function getTotal($records, $column)
{
    $total = 0;
    foreach ($records as $record)
    {
        $total += $record->$column;
    }
    return $total;
}

'weight' => array(
        'name' => 'weight',
        'footer' => $model->getTotal($model->search()->getData(), 'weight'),
        'headerHtmlOptions' => array('width' => 50)
    ),