Flames started out by being non-customized dual flames made only by a very very small group of people like Gman80. They were given for testing purposes to a very small group of staff like Macperfect. So at that point, a pair of dual void flames might have cost 300k.
When flame forging privileges were given to a dedicated "staff" group, the variety and uniqueness of flames went up, but so did their quantity. So outside of wing flames, prices dropped significantly.
When flame forging become something anyone had access to, you saw the steepest drop in price. It wasn't a vanity at all to own a flame. Nearly (if not all flames) don't even fetch for their forging price anymore. Whereas at least when the staff made a flame, they could ask for more than it's forge cost because only they could make them.
As time goes on, more and more flames are generated and flood the market. So it's inevitable that prices will continue to tank downward.