That is sort of what I was getting at with the first part of my response. There is no fixed rate that I'm aware off. It might even be worth thinking of the area your character finds him or herself and try and adjust accordingly.

I know, for example, that in ancient Egypt gold was more common that silver by a fair margin. You could trade huge amounts of gold for only a small amount of silver. If we extrapolate that to Althanas you might find Fallien (as the closest thing to Egypt we have,) has a much more even transfer of silver to gold than say Corone. Likewise Alerar might view gold as less or more valuable due to the huge dwarven mines. Perhaps mithril might be considered valuable and gold a mere trinket relatively.

I agree with the second part of your post. There is a big difference between the meta-currency value and how our characters would view gold.