Before February's run-up, Cavern of Souls had been stable for many months. There was really no catalyst for growth, and you can see that neither the buy nor sell prices moved much at all. However, with Modern Masters 2 looming large on the horizon and a guarantee that Cavern won't see a reprint (at least, not in MM2), it seems to have been pegged as a juicy target for speculators.
Though we know AVN won't be in MM2, there's a chance Cavern could appear in other sets, including Magic Origins. Now, I'm not saying there's a high probability that will end up being true, but I would say it's non-zero. The name on the card is generic enough, it's popular in Modern and getting expensive, and it's a fun, unique effect that would surely help sell booster boxes.
I asked QS staff writer Chaz Volpe to share his thoughts on this chart, as I know he'll be writing a bit about Cavern in his upcoming article.
I mention in my Insider article, Cavern of Souls will not be included into Modern Masters 2015. It's spread is something to note, as perhaps buylists are offering an attractive spread for incentive to sell, which may be attractive now, but could be an out-liner of the card to increase in price in a few months time.
The time-frame is never set in stone,
but most cards not included in a Modern Masters set usually see an increase across the board. Some increasing drastically, while others a slight bump. That being said, cards left out of these sets usually end up with a reprint in the near future after the set, but that's a chance I'd be willing to take. I'd say hold for now.
In all honestly, it would surprise me to see a non-promo Cavern reprint any time soon. I think your money is safe here. The spread has never been more favorable, but that leads to another question: does a narrow spread mean you should take your profits and cash out, or does it imply upside? In this case, my recommendation is to hold unless you have a better spec target that's cheap to buy into. I just see no downside in the immediate future, and there is a chance at upside. There's no downward pressure on the price whatsoever, and MM2 will certainly increase demand for cards like Cavern that won't be included in the set.
So, if you have some Caverns, are you buying, selling, or just waiting to see what happens? Let me know; I'm not sure there is a clear right or wrong answer here.