How much is an original french painting of Monseigneur the Duke of Brittany and Marie Adelaide is worth?

I have an original painting with the words "Monseigneur le Duc de Bretagne" (Monseigneur The Duke of Brittany) and "Fils de Monseigneur le Duc de Bourgogne et de Marie Adelaide de Savoye" (Wire of Monseigneur the Duke of Burgundy and Marie Adelaide of Savoye) on the back and i'm wondering how much its worth.