Yeah, the ampy cost is dependent on level, just like everything else that gets used in upgrade.
Honestly, the optimal way to increase stats with ampy units is to do it as low lv as possible
It was the same case for Sengoku Providence, which took a lot of inspiration from this game
I rarely pulled 6* girls. I think snowdrop, loquat, and, Fritllaria (might have misspelled it) are my only lucky pulls. The rest came from deals, rainbow coins, and forced 6* pulls.
Keep in mind, I have 21 rainbows now (5 of each type, Hit type has 6 instead)