Allosaurus Rider isn’t a bad card. Yes, it costs 7 mana to cast. But, it can have pretty high power and toughness, due to the fact they are based on how many lands you control. That’s OK. But you can also put the Rider into play by exiling 2 cards from your hand instead of paying its mana cost. That’s all well and good. It’s pretty awesome in a casual game, right?
Well, there are actually a couple explanations for the price spike. Eldritch Moon spoilers held the answers. The first is a mythic rare from the set with a mechanic new to the set.
Decimator of the Provinces (10)
Creature - Boar
When you cast Decimator of Provinces, creatures you control get +2/+2 and gain trample until end of turn.
Basically, what we have here is a “fixed” Craterhoof Behemoth. In some ways, though, this card is better. It’s all about Emerge. What Emerge essentially does is let you sacrifice a creature and reduce the Decimator’s casting cost by the converted mana cost of the sacrificed creature.
By “casting” Allosaurus Rider with removing two 2 Green cards from your hand, you then only have to pay 3 Green Mana to Emerge a Decimator of Provinces. That’s pretty nasty.
Is that the only reason for the Rider’s price spike? Nope.
There’s an even better one, and it's yet another Eldritch Moon card.
Because you can search out a creature with converted mana cost X or less, you can get any creature of converted mana cost 9 or less and put it straight onto the battlefield. And in Modern, what better choice is there to get than…
That’s right. A turn two Griselbrand.
Now how consistent is this combo? I have no clue. But that is, well, pretty good. Obviously, there are other things you can do with Eldritch Evolution. Still, you can't imagine too many people beating a turn 2 Griselbrand.
If an entire deck around this strategy emerged and became relevant, it would be pretty awesome. Grishoalbrand, a Modern deck built around Goryo’s Vengeance, Nourishing Shoal, and the Through the Breach, has been a fairly consistent winner in Modern. But Eldritch Evolution gives a whole new engine to build around. Allosaurus Rider seems like a silly target, and you probably won’t always have a copy in hand at the beginning of the game, or by turn two. As with any combo deck, there’s going to be variance. But this is the beginning of what could be a new archetype.
Of course, Allosaurus Rider didn't remain a $15 card. Decimator of the Provinces ended up being a pretty fringe card, sadly. Other Emerge cards did fare better, though. On the other hand, Eldritch Evolution has proven to be pretty useful in Modern. But despite the obvious combo with Allosaurus Rider and Eldritch Evolution, the seven-mana Elf just proved to be too inconsistent to be worth running in the deck. After a reprinting in the Duel Decks Anthology and falling from grace as soon as people realized he just didn't belong in Modern competition, Allosaurus Rider fell back to being a $2 casual card once again.
What we can learn from price spikes like this is that you just never know when a card will be printed that will make some obscure card sell out across the internet. Basically, if a card has any way that it can be abused, like with the alternate casting cost of Allosaurus Rider, it will eventually get its day in the sun... even if it's sadly short-lived.