There will be an independent quote of gold coin market averages that differs from the "official" gold spot price (which may have 7 years delivery delays and other hooks attached, etc).
Look at this from a different angle: assume legislative controls have been set in place, locking prices of all goods, and declaring no business may charge greater than a 5% premium on any good.
If the premium had previously been 10% on gold, then the price of gold will be the same as it was before (price controls), and the 5% premium would ensure that any dealers unable to stay in business at that level would run out of stock or close. In addition, those businesses remaining would be unable to acquire supply if any competition outside of the country were to arise, as having a frozen premium rate means room for negotiating on cost is non-existent.
Put simply: we have a choice of rising prices or increasing shortages. Price stability makes government look good; shortages make businesses look bad. Politicians can blame businessmen and assume greater control over industry.