The most optimal exchange is the 3000 pt one, which gives 10 iruka fr and 2 seal or 4 summon scroll.

The most valuable ninja frag is 10 pts, so it'd take 300 frag of farming to get that.

300 frag will get you 4 ninjas, that you don't already have, which will get you some coupons from collection quest. Thou at the same time, you might get those ninjas anyway through other sources eventually, most farmable ninjas are pretty common.