As I clarified in another post, I don’t expect folks to play out their economies in that way. Just that the short cut to make it playable doesn’t scale out to measure the economy of the world over time.
I also don’t expect there’s a particularly booming business cutting rubies, since they can’t afford the dust anyway. In which case some of the prove might be because the ones controlling the mining and cutting of rubies are keeping the dust for their own use in continual light spells, as wealthy people and governments might do.
Price is a result of supply and demand. If there were no demand for ruby dust, the price would be 0, it may even cost money to dispose of it. The default assumption is that supply and demand have settled on a price of 50 GP for the amount required for a PC to cast the spell. It may be a pinch of dust, it may be a bucket of the stuff.
As others have pointed out, most skilled workers could afford the 50 GP based on the scale set by the book. It could be a once in a lifetime investment since it lasts forever. Assuming you can find someone to cast it (or any other spell) I think they would be fairly common considering how useful they would be. I could see it being a typical wedding gift, or coming of age gift for middle class families for example.
To me, it's not an issue of the dust. It's a question of finding the person to cast the spell. That is what is going to vary from campaign to campaign and is going to be the limiting factor.