It is the difference between its price and the price of the fine gold contained in the object bought.

Exemple

1kg of fine gold is quoted at 8,500 Euros,

The theoretical weight in fine gold of a 20 francs napoleon is 5.806 grammes

The price of the gold weight of a napoleon equals:

8500 x 5,806 = 49,35 Euros
1000

If the napoleon is quoted at 51 Euros, its premium is 51 - 49,35 = 1,65 Euro that is 3,34%