It's probably the dub, and I mean it in several ways:
-By widely disemminating the "OP aims young" image, shops may price it assuming that parents might not be willing to blow eight dollars a book for their kids.
-Perhaps many dubbies won't buy OP tankos because they're lapped by the dub. Why spend money to see Arlong again when you already "know" the story? Bleach isn't dubbed, Naruto isn't dubbed so far along.
Or other theories:
-Perhaps the OP otaku (who would buy tankos of stories they've read) have, in unusually high numbers, subscribed to ASJ or bought original (or other translation) volumes or download scanlations. As a more resourceful group, they're less dependent on Viz products and therefore buy fewer copies.
Otaku are generally stupid, especially if they refer to themselves as such. The "average anime" guy/girl usually knows enough to realize that Anime is usually heavily edited, and what more could be entailled by a company called "4Kids"? Obviously, OP is not Hamtaro or Doreamon!
Also, I doubt the dub really has much affect on the manga readers. Just a feeling, really; A poor dub didn't stop the later explosion of DBZ popularity. OP just has a very unconventional style (interpreted by westerners as "weird"), such that it looks cartoony, so it's less Japanese than other Anime, and gosh, Japanophiles cannot like anything that's western! I mean, it's taboo!! -.-
We'll see how OP sells when Alabastia comes along. A more mature story might bring in a more mature audience.
Why would the stores do that? That does not make any sense.
It's the holiday season. Naruto has sold out, there's a couple Bleach volumes, and a shelf filled with One Piece. Doesn't that send some sort of signal…?
Was it the last day of the convention? Conventions usually lower prices on their stuff on the last day.
It was the last day of convention, aye. However, I still do not see why that would lead to a massive deviation in price between manga serialized in the same magazine.