بسم الله الرحمن الرحيم
It is better that you calculate it as per the market price and pay its fortieth part as zakah. For example, as a precautionary measure you should pay your zakah as per 3047.30. But if you pay your zakah according to the price in which your gold is sold in the market then too your fardh will be fulfilled, for example you evaluate it as per 2742.5.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.