I don't think the prices are more expensive than they should be. In fact I think they're quite cheap compared to the work you need to get them.
Time is a precious commodity, and getting heroic augments takes way, way, more time than it should. I would gladly pay the price for them instead of "working" dozens of hours. And I most certainly would not sell them if they were a lot cheaper.
Let's suppose grievous price was 10 million and the average time to acquire it (by playing the game and getting it to drop yourself) was 50 hours. Why the hell would anyone want to farm anything? I would just buy the 10m from someone with real money (should be about 5€) and buy the augment. 5€ vs 50 hours of work? It's not even a question, no one would want to farm... Of course in the end prices would still adjust themselves to the "real value", if it dropped this low and no one farmed anymore then there would be no supply and prices would rise... And would continue to rise until they reached the point where it made sense to farm again.
A player can't really manipulate the market by not selling something at the price he wants. In the end it's all about supply, demand, and effort required to acquire the items. I believe price is a natural function of these things and it always auto-regulates with time. If the price has been stable for the most useful augments, that's because it's their "real price" at the moment.
And this clearly has not happened with all expensive items. Take violence signets for example, 2-3 years they were about 600-700% more expensive. Supply has increased but demand hasn't, on the contrary.
Speaking of useful augments, ferocious, which is one of the most useful augments, used to be 15m each in the first month. But it seems obvious that the supply is much higher than the demand, so it's price was naturally adjusted to it's real value over time.