The docs clearly state that for a CAMPAIGN_PERFORMANCE_REPORT field "Cost" is type Money.
$user->LoadService('ReportDefinitionService', ADWORDS_VERSION);
// Create selector.
$selector = new Selector();
$selector->fields = array('CampaignId', 'CampaignName', 'Status', 'Clicks', 'Cost', 'Date');
// Report definition.
$reportDefinition = new ReportDefinition();
$reportDefinition->reportName = 'Report';
$reportDefinition->dateRangeType = 'LAST_7_DAYS';
$reportDefinition->reportType = 'CAMPAIGN_PERFORMANCE_REPORT';
$reportDefinition->downloadFormat = 'XML';
$reportDefinition->selector = $selector;
$xml_data = DownloadReport($user, $reportDefinition);
For v201309:
<row campaignID='1234' campaign='Cmp Name' campaignState='active' clicks='15' cost='40.24' day='2014-06-30' />
For v201402:
<row campaignID='1234' campaign='Cmp Name' campaignState='active' clicks='15' cost='40240000' day='2014-06-30' />
Please help!