$oCore_QueryBuilder_Select2 = Core_QueryBuilder::select('id'

->from('shop_groups'

->where('parent_id', '=', $Shop_Controller_Show->group);
$oCore_QueryBuilder_Select = Core_QueryBuilder::select('MAX(price)'

->from('shop_items'
->where('shop_group_id', 'IN', $oCore_QueryBuilder_Select2);
$row = $oCore_QueryBuilder_Select->execute()->asAssoc()->current();
$max_price=$row["MAX(`price`)"];
Работает.