I own two (EX Levi, Reg Eren) of the three currently released Sentinel AoT/SnK figures and had the third on order prior to cancelling.
If you're dead set on preordering, keep your eye on amiami. They're the only vendor whose discount is enough to warrant it, imho.
As for when it will go up, your guess is as good as anyone else's. This is the product page for him on the mfc database:
http://myfigurecollection.net/item/235530
I'd recommend checking that out regularly. Discussion over there isn't anything spectacular, but users almost always post when and where a product has gone up for order as well as the best deals to be had.
If, for whatever reason, you miss out, my advice to you is, don't sweat it. I remember back when Levi first came out, there was a false sense of urgency. Preorders sold out everywhere, and it was nearly impossible to find a place to get your order in. I actually ended up paying retail for him through Amazon.. Eventually the Japan release came around, and while he was still selling for retail (if not marginally lower), he was readily available.
Then the American release came around a month or two later, and the market was absolutely flooded. His price plummeted, and to this day you can still pick the exclusive up for a third to a fifth of the price if you look around enough. Well, I learned and managed to grab Eren, who also happens to be available for a third of his retail value, for $60 shipped.
Obviously this can all change with any given release, but experience tells me that I wouldn't worry about it too much if I were you.
Hope that offers a little peace-of-mind.
