The cheapest way for kushina hbh is great plates(the one before the last one we got), you could get her for around 5k or less there.

There's a very very very slim chance you don't get her completely in great plates(her drop rate is insanely high) but even if you do get that very very very slim chance, you get most of her frags and a decent amount of bp and you can just finish the rest of kushina's frags from free frags from events. Great plates gives you a way better run for your $ and even if you're that very unlucky person, you'll still get more value and will get kushina cheaper than if you spent in lucky dice to get her. About your statement on how great plates is only good to get bp, it depends how much are you willing to pour. If you were a guy like me who was trying to get lvl 9 in oil control, you'd get around 40+ frags of the other ninjas(in the old great plates not including kushina hbh), which would help you a lot if you were also trying to get those ninjas.

In that old great plates, the rng was rigged, kushina hbh had an extreeme drop rate. Everyone in my server who spent in great plates got her for 7k or under, so like i said before, you gotta be nearly impossible kind unlucky not to complete her if you spend at most 7-8k.