Plückbaum, from Germany, sells on 29 October 2016 as lot 1499 an "Antwerp Master, late 16th c., ca. 1600" portrait of Vincenzo Gonzaga, Duke of Mantua, estimated at 1,600 Euro.
Sotheby's sold this year a Studio portrait of Vincenzo Gonzaga for $37,500 . It was better than the one here though.
Looking at the picture, it seems to be one of the later portraits of the duke, so closer to 1610 probably. It should be worth 3,000 to 4,000 Euro.
UPDATE: not sold, contrary to what I expected. Perhaps people think it is a much later copy?