And to give a rough estimate, what total bonuses are needed to complete certain girls in less than a month

10 for Sanshi
1k for Dora
100k for Victoria
10M for Ashley

Judging by money needed only, there are clusters of girls you can complete with the same total bonus.

Cherry Lola Sanshi (Starting the game)

Ice Geeky Dora Frecly Pantress Victoria (factor in k)

Cindy Emma Indira (very high k)

Ashley Zoe Twins (factor in M)