I have no way of knowing without holding piece it in my hand, but that 8.5X11 at $14.99 is the equivalent of $21.60 per square foot. It is marketed by Tandy/Leather Factory according to the mark on the package. It is also probably cut from a side of import leather. The package or at least the clerk or owner should be able to tell what weight it is. If you buy a side of leather from say Wickett & Craig (utility skirting) it will cost you $100 for about 22-25 square feet, plus shipping, so you will wind up at a landed cost of just about $5 per square foot for USA leather tanned in the USA.
Now, having said that, since your intention was just to make one sheath, and IF the 8.5X11 is big enough to make that sheath, then I'd say go for it. If it's something you would like to do again, THEN start buying your leather differently.
Good Luck!
Paul