I actually happen to think the cost depends on the number the girl has in the harem. In my harem Abrael is the 20th, and Noemy is 15th. Even though Noemy's 4th and 5th stars came later, they seem to cost need less affection points and cash. Abrael on the other hand was crazy. So many affection points and money! I'm still collecting the cash to go up from 4 to 5. (19.4 mil needed to upgrade)

We need more heavy duty cash incomes.