Wild Nacatl is an updated, sharded Kird Ape. In the right deck, it's substantially better than Kird Ape, which is an eternal favorite for red/green aggro.
Wild Nacatl dominated its environment upon release. It quickly caught on in Modern after its release, in the mid-range aggro archetype Zoo, which dominated the format for a while. At the end of 2011, Wizards bans Wild Nacatl to clip the wing of the deck archetype, I suppose it made the games too quick to allow for more contemplative deck types.
Fast-forward to early 2014: Wizards un-bans Wild Nacatl in Modern, and the price, predictably, explodes. I grabbed one or two hiding in bulk after the announcement, after the spike occurred. I knew I had missed the boat on that spike, not having any market precursors by which to foresee. This is true in general, un-bannings are free-for-falls in the first few days, and a maximum price is reached early that is far above the equilibrium price afterwards. They do this predictably, on a published date. I might pay attention this year.
But I knew I had to watch this card, and wait for a good price.
After the excitement died down through 2014 and 2015, price is moving again, with some serious force upwards. I expect to see these to surge well through $2.00 by Spring 2016, maybe a bit of a trough during 2016, then another surge up to $5.00 or so by then end of 2016 unless we see another reprinting. Modern Masters 2017 (if it happens) would be the obvious choice for that, so move these before that time. In a way, Modern Masters 2017, still only presumed at this time, is a bit of a reprint reprieve for this card. Now is a great time to get in, you have well over a year to see price growth.
These are used 4-of, usually, so the demand will continue, thick.
The promo FOILs and the regular FOILs are both around $4.00. These are a great pickup: expect them to be eternal favorites, with price tags of $20 or more 2-3 years from now.