If there becomes a deck that wants 4 copies of this version of Sorin, his price should see even more growth. Right now, decks are only playing one or two copies of him, which is keeping his demand down. But as a mythic rare from Khans of Tarkir, his supply is around the peak right now. Trade for them or buy them on the cheap when you can.
This is a planeswalker that should find a permanent home in Blue/White or Esper Control. While she's seen play in these archetypes, as well as Jeskai Tempo & Token decks, the Transcendent hasn't gained widespread acceptance as a key part of a top deck. There's too much aw power here to stay around $10. Definitely one to target as a trade pickup or a good place to put your store credit.
As the Modern format grows, it's likely to see the Coronet grow in value, as well. It's not out of the realm of possibility that this version will eventually gain enough value to match the $15 or so value of the original printing. If you're looking for a good Modern pick-up, this would probably be it, There's use for it in Commander, too, so unloading copies shouldn't be hard at all.
With rotation coming up in October, it's possible his price could drop a bit more.. Still, $10 seems like this card's floor. His casual appeal alone could keep him at that price. Long-term, his token generation, 3/4 Vigilant body, and popular Tribe will make sure he has the potential to at least double his current price in the long-term. This may be the best time to pick him up. Eternal playables, especially mythic rare ones, simply don't stay this price.
In general, Modern Masters 2015 cards are at pretty low prices, It's probably the best time to get into any cards from the set that you want.