Here is top 5 girls with HIGHEST income and also HIGHEST upgrade cost (Before lv 100)
1. Cassandra (The Amazon - Unlock at Company Lv 6)
2. Judy (The Punky - Unlock at Company Lv 10)
3. Laura (The Stepsister - Unlock at Company Lv 7)
4: Anika (The Nurse - Unlock at Company lv 8)
5: Delilah (The Dancer - Unlock at Company lv 9)
And here is the same list BUT After lv 100:
1: Cassandra
2: Laura
3. Anika
4. Delilah
5. Judy
So as you can see, their cost increasing after lv up with Cassandra and Laura have highest cost increasing, Judy start with high cost to upgrade but after lv 100, her upgrade cost stay lowest
When you are at lv 8, you will easily upgrade girls to lv 100 so the second list is useful for that and REMEMBER these girls have the same income and highest among gold diggers
Here is example: i have Laura, Anika, Deliah and Judy at lv 200. Judy need 713Sp to get to lv 225 while Deliah need 1.110c, Anika want 1.370c and guess what: Laura need fucking 1.650c. Laura upgrade cost is multiplier by 2.3 compare to Judy in exchange for 25 lv