How to minus number in DB

PHP Code:

  function minus_set($id)
  {
    $this->db->trans_start();
    $this->db->set('total_set', 'total_set - 1');
    $this->db->set('price', 'price - 300.00');
    $this->db->where('user_id', $user_id);
    $this->db->update('users_detail', $data);
    $this->db->trans_complete();

    if ($this->db->trans_status() === TRUE)
    {
      return true;
    }
    else
    {
      return false;
    }
  } 


This is my model. I want subtract the total set and price. But, its not working.. how to fix it?
thanks

This is me. JK not me.