This event is one of the cheapest to but Sage Naruto, also you can count on 11 free keys daily.

77*3=231 points (in the worst of case)
So you can take almost 10 frags for free.

Then you need 70 frags more to take him:
1750/3=584 keys (always in the worst of case)
At max Sage Naruto will costs you 5840 coupons, probably less :).