Gold and silver are commodities that are actively traded on the world markets. As such, they are subject to the market forces of supply and demand. This causes the price to change over time. Typically, in times of international insecurity (i.e. wars, changes of governments in world powers) the prices of gold and silver will increase as people purchase gold and silver to use as a trading medium in lieu of paper currency. Additionally, the price of gold and silver paid by industry is higher than the published bullion rate. This is because users of the metals must pay a fabrication and distribution charge, which is added to the bullion price.
As precious metals costs change, this is reflected in the price of the plated component that consumes the precious metal. This is typically changed on a monthly or bi-monthly basis (weekly or even daily in times of rapid change), and the base price of the gold and silver is printed on the bottom of the invoice. Advanced Plating Technologies bases gold and silver invoicing on the Engelhard Fabricated indexes which can be found on the Wall Street Journal commodities page.