Ajax Trigger module allows you to do specified actions on the server on JavaScript events on specified fields.

Example #1 - Calculating price in real-time using hook_ajax_trigger

1. Active Javascript Events for specified Field on Field Settings Page (admin/content/node-type/party/fields/field_name)
2. Example code:

/**
 * Implementation of hook_ajax_trigger().
 */
function foo_ajax_trigger($form, $field, $op) {
  $form_id = $form['form_id'];
  if ($form_id == 'my_form_name') {
    // Load required values from form
    $price = (int)$form['field_foo'][0]['value'];
    $div = 'Calculated price: £'.number_format($price,2);
    $code = '
      if ($("#price").size() == 1) {
        $("#price").html(\'<h1>'.$div.'</h1>\');
      } else {
        $("#edit-submit").before(\'<br><div id="price">' . $div . '</div>\');
      }
    ';
    return $code;
  }
}