There is no problem with NLS_LANG of client, client is using either WE8ISO8859P15 or WE8MSWIN1252.
Even if I viewed from sqlplus of database server, the same upside-down '?' is shown.
If I query with sqlplus to another database which has no problem with euro sign, the euro sign symbol is displayed either blank or black rectangular, due to bug of sqlplus. With my database which has this euro problem the euro sign is displayed as upside-down '?'.