Well she's pretty much the same price (costing about 40 coupons) in lucky board and treasure of sage. The only difference being that you'd get a LOT more stuff as an extra for getting her in lucky board rather than buying her straight up from treasure of sage.

i currently have 640 coupons to get the 16 frags i need but the thing is she needs to be at 20% discount so i can get those frags

Sailor sakura is a very good ninja to get, and even a good ninja to 4-5 star with events. Since she's usually cheaper than most other ninja in the event.
It's a ninja that easily finds a spot in most teams (except blitz) if they need some healing. So i reccomend getting her yes. Even free to play you can get her easily, since she shows up in alot of events.

She can appear at 30% off with 35 coupons a frag :)