Have a Sevres porcelain plate circa 1840 with a hand painted portrait of the Duc de Bourgogne on it It has the royal mark on the back and the mark of the Chateau de Tuileries What is it worth?

I can't tell you the value, but I can tell you how to find out. There are collectors books that list the secondary market value of plates and other items. You can look in one of these books under the makers name/brand and it will list the plate by name and date. Since yours is so old it may not be listed. Just the fact it is an antique makes it worth something to a collector of plates. The value for a collector is determined by how good shape it is in. If there are any chips or cracks in the plate or if the picture on the plate still has its colors and there is no fading the value goes up. How rare the plate is may determine value. You could take your plate to an antique dealer to see what they offer, but be careful they will offer you less than its true value.