A rough estimate if the case back is 34mm across and (estimate...) 0.5mm in depth, it would be about 0.45 cubic centimeters of gold. At 15g per cubic centimeter (for 18K gold, not "solid") that would be about 7g. My suspicion is it might turn out to be less than that. Probably easiest to weigh it.
1g of 18K gold fetches about £35 today. It will be less for jewelry that needs melting down.