If the buyers think the price is reasonable and affordable for them then they will buy, if not, they won't. Just that simple. These items/mods ain't required to progress through the game so it all depends on the desire of the buyer. And i don't think there will ever have a "professional source of statistics" to price a riven mod because almost every riven mods are different than each other. I am not saying these prices are reasonable but it is also subjective to say. Auction house may provides more choices but i don't think it can solve the riven mod's "overpriced problem".
Tradeable items in warframe are mostly prime warframes and prime weapons. They don't work like the equipment in other games. In most of the other games, once you level up to a certain level you can change to a better equipment so there will be a clear price stair. However in warframe, its all about mix and match which makes the price stair of item rather blurry than others.
If adding a auction house in warframe, we all know that all item prices will have a significant drop since most of them don't have a very strong price base and then auction house will be flooded by many items/sets which no one willing to buy since the supply will be way higher than the demand (thats why there is a ducat system for you to burn the useless and technically not sellable prime item). In a pool of the same item, sellers will tend to sell for a lower price and then platinum becomes less valuable. I don't think DE will destroy their main income in this way.