No need to concatenate a php variable when already opened double quotes try this
$query = $this->db->query("
select max([$product]) as price from Price where customeraccount='$customer'
");
or
$query = $this->db->query("
select max($product) as price from Price where customeraccount='$customer'
");
While about the error you are getting is due i think your database driver is not loaded first try to load database
$this->load->database('default', TRUE);
The best way to use CI's Active record you can do so
$this->db->select_max($product);
$this->db->where('customeraccount', $customer);
$query = $this->db->get('Price');
See Active Record