product.set.price.php

<?php

switch ($modx->event->name) {

    case 'msopOnModificationSave':
        $modification = $modx->getOption('modification', $scriptProperties);
        if (!$modification) {
            return;
        }
        /** @var msProduct $product */
        $product = $modification->getOne('Product');
        if (!$product) {
            return;
        }
        $modx->invokeEvent('OnDocFormSave', array(
            'mode'       => modSystemEvent::MODE_UPD,
            'id'         => $product->get('id'),
            'resource'   => &$product,
            'reloadOnly' => false,
        ));
        break;

    case 'OnDocFormSave':
        $product = $modx->getOption('resource', $scriptProperties);
        if (!$product) {
            return;
        }
        $price = $product->get('price');
        $q = $modx->newQuery('msopModification');
        $q->where(array(
            'rid'    => $product->get('id'),
            'type'   => 1,
            'active' => 1,
        ));
        $q->select('MAX(CAST(`price` AS DECIMAL(10,2))) as price');
        if ($stmt = $q->prepare()) {
            if ($value = $modx->getValue($stmt)) {
                $price = $value;
            }
        }
        $product->set('price', $price);
        $product->save();
        break;

}

создать плагин с данным кодом. Назначить два события * OnDocFormSave* и * msopOnModificationSave*