It costs 5 times advanced refinements, 3 times orochimaru akatsuki and 20% more than a charm pack. This means cost 170-180 coupons per fragment, and, infact, the average point you get out of 5 sacks is 7-8. I can confirm this since every time this event appear i am just barely able to buy 2 charms packs out of free points and thw free sacks are 35.

The average cost is 7500-8000 and given the number of sacks needed the sd can't be higher than 15%