Multiple table prepare() query in xPDO

<?php
$minPrice = $modx->db->query( "
    SELECT      
    MIN(item.price + box.price) AS sumPrice
    FROM     `item_table_name` item, `box_table_name` box
    WHERE    box.width  > item.width
    AND      box.length > item.length
");

<?php
$item     = $modx->getTableName('Item');
$box      = $modx->getTableName('Box');
$iCols    = $modx->getSelectColumns('Item', 'item');
$bCols    = $modx->getSelectColumns('Box', 'box');
if ($stmt = $modx->prepare("
            SELECT MIN({$iCols} + {$bCols}) AS sumPrice 
            FROM {$item} item, {$box} box 
            WHERE box.width > item.width 
            AND box.length > item.length" )) {
    if ($stmt->execute()) {
	$output = $stmt->fetch(PDO::FETCH_COLUMN);
	$stmt->closeCursor();
    }
    else $output = 'query failed';
}

return $output;

<?php
$prefix = 'modx_box_';
$packageName = 'box';

$packagepath = $modx->getOption('core_path') . 'components/'.$packageName.'/';
$modelpath = $packagepath.'model/';

$modx->addPackage($packageName,$modelpath,$prefix);

$c=$modx->newQuery('Box');
$c->select($modx->getSelectColumns('Box', 'Box'));
$c->innerJoin('Item','Item',array('Box.width  > Item.width','Box.length > Item.length'));
$c->select('MIN(Item.price + Box.price) AS sumPrice');
$c->select($modx->getSelectColumns('Item', 'Item','Item_'));

$c->prepare();echo $c->toSql();
$object=$modx->getObject('Box',$c);

print_r($object->toArray());

<?php
$c = $modx->newQuery('Box');
$c->innerJoin('Item','Item',array('Box.width > Item.width','Box.length > Item.length'));
$c->select('MIN(Item.price + Box.price) AS sumPrice');
$c->prepare();

$object = $modx->getObject('Box',$c);
$sumPrice = $object->get('sumPrice');

$c->prepare();

echo $c->toSql();