Hi Pellinore,
the weight of goldcoins depends on their worth, I think. Bigger coins are worth more but heavyer while smaller goldcoins are worth less but lighter.
A good way might be to do it as they did in former times: Some things are worth their weight in gold, others are worth perhaps a tenth of their weight.
To simplify the whole matter one could say a thing is worth xxx grams gold. 1 gram gold is worth xxx grams silver and so on.
Are you really playing since the 80's? Then you are even more a RPG veteran than me
Best wishes
Keltoi