that actually is a common misconception as affection and upgrade cost increases with increasing number of girls as well (well unless you have 100+ girls), to simply get even - moneywise - with what you have to spend to upgrade a girl it takes month-years (depending on affection level and girl), unless you have 100 girls you dont actually speed up the upgrading process for said reasons as a single girl does not add much towards your total income (~1.3k/h, which translates to maybe 13k/day if you are pretty active - 390k/month, 4.7kk/year - that is not even what you paid for 10k affection+upgrade costs)
maybe you do speed it up, but if you do it is literally not noticeable