To me, it is currently worth it to use 1* Manyus on leveling a high level girl, but only to get her to the point to evolve her, not to get her to max level after evolving. Mostly because I got a lot of gold last week, but very few manyus.

It was bad enough that I even experimented with leveling girls using a variety of dupes. Above roughly level 15, leveling a girl using dupe cards just isn't worth it. The one 4* dupe (Lilac) I had available seemed to give about (just an eyeball estimate, not exact) half the amount of experience of a 1* Manyu.