Was there ever a set value? It's supply and demand from both sides. Right now you have a lot of players playing a lot, so there's a demand for game time that exceeds the need for gold. There's more gold floating around than there is stuff to spend it on, unless you're in a high end guild buying all the expensive raid buffs. Even with runes and pre-pots I find myself with a surplus of gold and I really don't put much time or effort into making gold. So I'm sure the players who do have even more.
If we get to a point where more players need extra gold than want free (in real life money) game time, relatively speaking, the market will drive prices down. Right now we're just the opposite.