It's not too bad, since I mostly level whichever girl of a given type happens to have the lowest attack. So sometimes that's a lvl 58 girl about to evolve, other times it's the most recent event girl or a girl I evolved just the day before. Overall, the costs avarage out, and I'm slowly improving all of the girls I have. It's not a logical strategy for upgrading, but it works for me, since I mostly settled on the girls I actually have on my team, and those have already reached lvl 70 or 80 and only occasionally get ampies when I have them.