Заменить:
$price = 50;
if ($url['get'] == 'price')
sys::outjs(array('sum' => $price));
на:
$price = 50;
$days = isset($url['params'][0]) ? (int) $url['params'][0] : 1;
$sum = $price * $days;
if ($url['get'] == 'price')
sys::outjs(array('sum' => $sum));