I guess it depends on what you're trying to accomplish. Kerdi is Kerdi. Schluter makes Kerdi "accessories" which are Kerdi-Band and Kerdi-Kereck. These are custom pieces of Kerdi to help with corners, seams or round openings.
I pay around $18 for the Kerdi-Band you're talking about but there would be a shipping charge on that. All in all, $25 is not outrageous.
The difference between regular Kerdi fabric and Kerdi-Band is the thickness. Kerdi band is half the thickness to minimize thickness build-up when we treat the corners. You shouldn't compare the cost of the two items, although you can cut regular Kerdi instead of buying the Band.
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