IIRC, the original last 5 girls still have the highest (and the same) income per second. But the last girl (Judy) is cheapest to upgrade. Cassandra is most expensive. (but why use her when Judy is cheaper with the same income)
Annoyingly, for some reason only known to the devs, the 3 new girls were randomly dispersed into the hire list in no order that makes sense to me. But none of them give as much income as the original 5 last girls.