SELECT SUM(precio) AS totalprecio FROM productos

<?= yiigridGridView::widget([
    'dataProvider' => $dataProvider,
    'showFooter' => true,
    'columns' => [
        ['attribute' => 'precio',
            'footer' => sumarizeFooter( $dataProvider->getModels() ),
            'format' => 'raw',
        ],
    ]
]) ?>

function sumarizeFooter($model){
    $iSum = 0;
    foreach ($model as  $obj){
        $iSum += floatval($obj->precio);
    }
    return $iSum;
}