Mantra‘s first series had three significant alternate covers (there is also a price variant for No. 2 when it is in a polybag; I’ll cover that in an upcoming post).
Of these variants, the one above on the right is far-and-away the toughest to find. I’m guessing from the UPC box that it was a news-stand book and therefore wasn’t distributed in comic shops. In any case, you almost never see these. I bought mine from Joseph Koch’s online store for, I think, $3.
On the back of the book on the right is the regular cover, kind of like a flip-book; the one on the left has an advertisement for the Super Advantage SNES controller.
The UPC variant for No. 8 is a little easier to find. The books without UPC codes are common, quarter-bin stuff. I would pay a buck or two for any of the alternate covers I see, though.
With No. 13 both versions are pretty common. I’ve snared a few of these as parts of a larger lot of books. The most you should pay is a buck for each on a place like mycomicshop.com.