Thank you for your excellent info, Bron!Hi, Cameo brooches have been in production for over a hundred years.
Sometimes the cameo is made from agate or carnelian (one carved piece of or two) from shell, man made materials ie Bakelite and even plastic.
The value is usually determined by the quality of the cameo carving, size, uniqueness/ visual appeal of the scene / ladies head and also the metals used. The metal can be solid gold, gold plated or filled base metal, a material called pinchbeck and just gold coloured metal.
Solid precious metal will be hallmarked. If the piece isn’t hallmarked it is likely just a gold plated or gold colour base metal.
So depending on all the factors prices can range from $10 to $10,000.
If a shell cameo set in 9kt gold around $300, in 18kt gold around $600, if not hallmarked, less than $50. Other natural materials and embellishments ie diamonds / pearls/ enamel can add $$$ to the value.