On long run N-Gacha have this chances without card reveal
SR-Girl 1,5%
R-Girl 20,0%
N-Girl 78,5%


So if all you N-Girls are maxed spending 315.000 Pero (15k goto combine) on N-Gacha (1500 Pero each) would on average give:
55 R-Girls level and 3 SR-Girls levels.

Spending 333.000 Pero on N-Wild + combine give:
3 SR-Girl levels. Here you can chouse which girl you want to level.

I never buy N-Wild. I save my money for when I really need N-Gacha or when there are special offers like N-Gacha + N-Key for 1500 pero.