How are people going to know if it's real gold?
I think that how much it will be worth depends on the value people assign to it.
As to authenticating a gold or silver coin, I would bring up a 2,300 year old answer to this question which is--interestingly enough--the source of the word "eureka" that people shout when they have an inspiration or a flash of genius.
Archimedes of Syracuse was friends with the king of a city-state. Archimedes had a reputation as a mathematical genius (he was, by the way), so the king approached him with an interesting problem.
The king gave a specific quantity of pure gold to an artesian to fashion into a crown. Of course the king weighed it when he got it back (he wasn't stupid), and it weighed the same, but what if the artesian stole a quantity of gold--say 15%--and replaced it with an equal mass of much cheaper copper? The king could be cheated without ever being aware of it, and this was a concern.
While Archemedes was pondering this, he went to the public baths to relax, and noticed the water level rising in the pool when he settled himself into it.
This caused him to run naked through the streets toward the palace, shouting "Eureka!" over and over again, which means "I have found it!" in classical Greek.
So, if you know the density of pure silver and pure gold, you can measure the volume of a coin in a graduated cylinder and figure out how much water is displaced. You can then weigh the coin on a precise digital scale, and figure out the density. If it matches what you know the density of gold to be, then you aren't being swindled.
This test takes very little time, and is not destructive to the coin.
If I remember correctly, the artesian was executed . . . but I could be wrong.