Откройте /controllers/admin/AdminProductsController.php и до:

$this->fields_list['price'] = array(
            'title' => $this->l('Base price'),
            'type' => 'price',
            'align' => 'text-right',
            'filter_key' => 'a!price'
        );

добавьте:

$this->fields_list['wholesale_price'] = array(
    'title' => $this->l('Wholesale price'),
    'type' => 'price',
    'align' => 'text-right',
    'filter_key' => 'a!wholesale_price'
);