getting frags for free each week is maybe better choice then spending on her. She is in events every week,

But in my opinion cheapest event is maybe SAkura (one with food boxes ).. Kushina was there few times for around 10k for 80 frags. But she is not there this week. You can get 14 frags for free from Missions General Mobilization. Getting her for free like this will save you a lot of CPs at end if you decide to buy her

3 --> 4: 1400 pts (1400 Ramen) [14000 Coupons/Ingots])

Thats the price for Kushina in Naruto Ramen event. And i know i saw her for 10k or less.

But as everyone got different answer on your question you c*ways join one of the Naruto discord and check prices there. They post prices every event cycle and they are never deleted. So you can compare prices for last few months. But difference in price is not that huge.

But when ever she is up in Naruto Ramen or time limited shop you c*e your extra stamina pots to gain extra ramen/limited points. i got my kushina to 4 stars without spending any CPs. Just be careful because you can get only 20 ramen per day. And there is no limit on how many limited points you can get. Just save ur stamina pots untill then.