You need to change price scope 'Global' to 'website'
(Sysytem->Configuration->Catalog->Catalog->Price)
$client = new SoapClient('http://your-web-site/api/soap/?wsdl');
$API_USER = 'your-api-user';
$API_KEY = 'your-api-key';
$session = $client->login($API_USER, $API_KEY);
$result...
