For Kakuzu case the gold vialing rng is a bit hard to get Standard attack +1 target and the double attack so the cost will increase,I saw some people rather going for another 100 fragment of kakuzu to unlock the double attack.Quite costly ,but he is an one man army if you manage to land of his important trial skill.

For Naruto(Six Path) case he is cheaper and useable right off the bat at 3 star but i do not recommend on getting him at this point of the meta (I might be wrong),people still use him but that is for player who bought him before Kakuzu/Kushina Bt and Trial were introduced into the game.At this point i feel like it would be a little bit of waste of buying him.

Six paths is still playable. He is a cheaper version of kakuzu. The name of the game right now is super stacked position 1 and being able to one shot your opponents position 2 to 4. Kakuzu is the best for this task. His standard is AOE along with his chase not to say he can STandard attack twice. Naruto can do the same as well with killer bee, he can standard attack twice. Between the two Kakuzu is the better ninja. He doesn't need other ninjas to scale because he scale through chakra.

Kushina is a tricky cat. Her lack of AOE is a problem but once you are in round 5 and she is alive, you will win the match.

Ballruto if below 350k usually, cuz most people using him are around 350k avg, the only people i seen using him around 300k are people who are waaaay higher on pos1 compared to pos2.