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.