Given that we have to wait for the BT to have him show up in more events, the only event he shows up is in fateful choice. So you can't really ask the cheapest price atm since he shows up in one event.

Also, do yourself and join the discord server if you haven't. This is what the latest prices on Naruto in Fateful:

Unlock: 11680 points [12848 - 17520 Coupons/Ingots]

3 --> 4: 14600 points [16060 - 21900 Coupons/Ingots]

4 --> 5: 29200 points [32120 - 43800 Coupons/Ingots]