Hello!

Kushina is decent at 3 stars, although at 4 she starts to be good. For her breakthrough, you would need 750 fragments.

Imo you should wait for her Skill Trial to come, to reduce the cost. If you want an approximation, if 100 frags are about 13-14k(depends on the event) then you would need around 100k. As far as breakthroughs go, you don't need all of them, just the Chase and the first Passive.