The rise of Modern Eldrazi on the back of Oath of the Gatewatch's envelope-pushing power and the discrimination of colorless mana into "true" colorless mana and generic mana has focused attention on the most-printed non-basic rare lands in Magic history: The Pain Lands.
That is, Underground River, Adarkar Wastes, Sulfurous Springs, Brushland and Karplusan Forest.
These five lands were first printed in Ice Age, and have had many reprints since then. Their enemy-color bretheren didn't arrive until Apocalypse (Shivan Reef, Llanowar Wastes, Caves of Koilos, Yavimaya Coast, Battlefield Forge) and have seen more recent reprintings than the original five (Shivan Reef and company just got a reprint in Origins: these are fully deflated and on their way back up, too).
You may have noticed this trend over the past couple of months. I saw it, and expected slow and steady gains, and grabbed a small holding of pain lands. As always, 10th Edition is the most sought after of the bunch. But the speed of this movement took me by suprise.
A few months ago, I would have suggested that $6 was about the ceiling for Pain lands, forever. But a couple weeks ago, Adarkar Wastes changed the rules for pain lands, forever:
Karplusan Forest popped the same way at the same time and is up around $10 now. You can see that this still has some momentum after the spike, respiking well over $10. This is real, this price will stick. Sulfurous Springs seems to have moved already, and stablized around $5.
This leaves Underground River and Brushland: these are your best bets currently to grab ahold of this trend. They will burst imminently. Foils (7th or 10th Edition) are my recommendation for highest yields. Check this out:
If you can find ANY of these for under $2.50 apiece, even beat up 5th Edition copies, they're a good pickup. NM 10th edition copies are worth speculating on at $4 apiece.
These will be reprinted, and probably soon. The enemies just got a reprint in Origins, so I suspect that it's a matter of 6-12 months, not years. I don't think they will ever see $2 per card again, however. $4 is the likely bottom if reprinted at Rare.
I'm using these as short-term specs, as I already have plenty of play copies for myself.