A card's value typically stabilizes around how often it is seen played in competitive decks. The only deck currently running it is a standard four-color Eldrazi ramp. It's not seeing solid play in any other competitive format so far, which explains the sheer drop in value. For EDH/Commander players, however, picking a copy(or more) of this monstrosity up may be a great idea, because chances are the value of this card will skyrocket in the long run and bolster your binder.
When Consecrated Sphinx was spoiled and finally released in Mirrodin Besieged, it was valued at around $6. While there have been some dips back down to around that number, Sphinx has been slowly yet steadily on the rise, and currently sits at around $23-and what it does is incredible. In a 1v1 game, Sphinx nets you two extra cards on your opponent's turn and has evasion, making it useful for combat as well. In a multiplayer game, Sphinx can be the devastating cascade for a player to win the game, drawing an insane amount of cards for the investment of 6 mana.
How does Sphinx compare to Sire, though? By all accounts, Sire puts in more work for the investment in Dimir (U/B) rather than just blue-which may also attribute to its low price. Everyone typically has to draw a card during their turn, which will net you two cards of your own with Sphinx-but playing land is a voluntary necessity, and the steep cost Sire makes players pay will make them think twice, watching their deck wither away at the price of trying to win the game. Late-game mana drops for your opponent(s) become painful as their decks begin vanishing right before their eyes. This kind of low-key control strikes fear into players desperately trying to get more mana. You also net two extra cards from their land drop, giving you a distinct and substantial advantage.
Considering that their converted mana cost is the same(though Sire requires U/B rather than UU), and Sire's stats are better(a 5/7 compared to Sphinx's 4/6), which only balances out because Sphinx has evasion, Sire's level up is that it exiles two cards off the top of your opponent's library, milling them while you gain card advantage. No need to scry or delve into the issue further-if you're running U/B in EDH/Commander, grab a couple of these now while they're cheap, before a frenzy for them drives up the price!