Ancalagon
Dusty Dragon
You kid but... any economic resource can become a major plot device, and that can be used in an RPG campaign. Light is valuable after all (even for darkvision-having races, it only gives you dim light equivalent, ie you can't see fully). After centuries of use, perhaps the world is running low on rubies...the real money from the continual flame casting business is in grinding up 5,000 gp rubies and selling the dust... 200 50 gp bags.
The spice must flow... or perhaps, the ruby dust must.