Stupid question, but...
Are you sure? The last 4 girls for me give exactly the same amount of money per level, with the exact same upgrade items, and I'm pretty sure they always have.

(though I was unaware "story" affected price, they are all 1/4 on the 3rd panel. (just far enough to unlock nude private viewings. After that I've been going through "in order" as far as timed unlocks allow.)