It's not 10, 20, 50, it's 10, 30, 80, the latter reward INCLUDE the earlier, so the overall price per frag is LOWER.
Or, to put it differently, it would cost 23500 coupon to get 80 frag hidan in ONE event.
It would cost 4200*8=33600 coupon to get 80 frag in EIGHT such event, getting the 10s only. Ergo, going for the BIG one is better.
Edit: PS: The only "rebate" event that I know of that gives LESS per unit of spending as total spending increase is the cave one (when converting all reward to coupon value). Granted, I did the calculation during the previous one and didn't bother this time around, if prize changed, it might not longer be true. Summoning rebate is slightly weird, as 20 have better either scroll or rune reward ratio than 50/80 (thou the other reward, rune or scroll, is better on 50/80), but in the very least, the 100 is strictly superior th*l others. Alas, I do in fact calculate the rebate return % on pretty much all such event. On that note, if you don't have much seal scroll, skip this seal rebate event. If it's like the previous ones, it strictly inferior to the specific treasure seal rebate event (less reward for same spending AND temp scroll instead of real ones)
This post was last edited by 117***@google at 2017-3-29 11:35