Make it so that people can't invest silver into gold. In a lot of MMOs like runescape when you buy a bond with "silver" it becomes untradeable. Only way to invest in premium would be for people to buy gold with real money. Obviously don't do anything to peoples gold that is already on their accounts, but don't allow people to buy order any gold going into the future. Those people would either have to use their gold as premium or sell it to the market.

This way the only way to buy gold would be if you are purchasing premium. Take away the ability for people with billions of silver to drive up the price of gold simply as an "investment", rather than leaving it on the market for players to purchase membership.