After save, $data->id
should be the last id inserted.
$data->save();
$data->id;
Can be used like this.
return Response::json(array('success' => true, 'last_insert_id' => $data->id), 200);
For updated laravel version try this
return response()->json(array('success' => true, 'last_insert_id' => $data->id), 200);