Just taking a stab here, on leather you will need enough to cover all4 sides of the object to be carried + material for attaching to your belt. Just a guess at my local Tandy leather for 1 holster will range about $40 or so, depending on the thickness you want.
The sewing kit from Tandy should cover your sewing needs, if there is not an Awl in the kit my advice is get one, during the sheath calss I took while not using an awl, I broke 3 needles. Also look at the color of thread in the kit, you may want a different color for your holster.
Just my two cents. Good Luck.