The formula is basically the same thing I wrote myself and he/she quoted.
The formula is true for first 5 girls (checked on alt-account).
Not sure about more girls than that - all my girls are 3 stars - so to check/calculate/etc. I will need to wait for kobans/event/story quest.
Problem is they pushed "waiting" from actually buying premium content with premium currency to upgrading/unlocking scenes with cash (non-premium currency) or kobans (so premium currency).
That cost of upgrading goes up the more girls you have - AKA the more you play, the more you support the game, the more it will cost to support it next time.
Just imagine you play an MMORPG game like WoW where you need to pay monthly subscription and the monthly cost goes up with each month. Thats ridiculous, bad design, borderline retarded ;]. But thats what basically HH devs gone and made with their changes.
I understand that they wanted to make something so that F2P are incentivized to support via actually buying kobans, and keep old whales, but you don't do it this way.
Good game design: gameplay is fun, premium stuff is cool/sexy/etc. - game itself makes you want to support it cause its fun. When you buy premium content you get premium content and its not hidden by another wall/etc.
Bad game design: game has slow story updates, broken PvP, you "prolong its life" by making changes in some systems introducing a slopped rising cost requiring a tedious boring busy-work or spending more and more of premium currency. Premium content is hidden behind double wall: the same RNG based gacha/pachinko like other games (thou in some circumstances you have far more chances to get what you want in HH), and now a secondary wall of unlocking starts that cost more the more girls you have.
Thats not a way to prolong the life of a game, its way to slowly kill it. I seriously would like for a dev to came and talk to us in this forum. Tell us if they are just that - for the lack of better word - incompetent, or did they do that because they are bored of this game and want to slowly kill it.
My advice - add seasonal resets of tower leaderboards. Do something so that you would be able to release story more often. Make it so that first 4 girls costs to affection upgrade are 20/40/60/80% sloped values of all other girls - and all other girls will be for example costing as much as Juliette (so 2nd girl) is now.
Fixed PvP will make it so that new players can actually fight for it, and have incentive to level faster, to get better items, to level best girls for their character in their harem. As its now - no sane person will even try to compete cause its not a PvP per se now - its who played longer and clicked a button more times.
Story is released to slow - we basically need to reread older quests when we get a new one, cause we already forgot what was happening there. Thats fucked up. Heck even 30+ paged chapters of mangas are released more often.
Fix to "broken by the 2.0 updated" affection. Will still have higher cost of upgrading that will force F2P people to work for their affection scenes. While the starting girls will be cheaper and provide a quite fast slop to the static bigger cost. It will be cheap enough for whales to try to insta unlock them stars. Beside with new level cap, we already have slowly rising costs of upgrading our stats, and rising cost of getting better and better gear, we don't need yet another - frankly ridiculously fast - rising cost in the affection section.
Welp. Yet another big wall of text from me. Question is am I just retarded and write the same stuff over and over again, or am I that passionate about some game I play when I see devs fucking their shit up ;]
EDIT:
As a side note: At level 46 I got again 50 more upgrades. So the theory about static 50 upgrades since level 14 seems more and more true xD