Basically it's like DuckSwim described.
With the way I'm doing things now, it's even a bit more complex when you look at the details.
Not everything has to be done for all girls, that's true. But it will be done for at least most of the girls over time, so the cost is the same (or even higher, because of additional setup times).
Not everything has always to be done for three outfits. If e.g. spa can also mean sauna (which is part of the spa), there is only one outfit, meaning naked.
On the opposite side of things, a render count of three is no longer realistic for any new scene. It was in the past for really simple stuff. But now, I can't remember when I last did any scene that had less than 8 or 10 renders. If you take that into account too, it means a lot more renders.
Also the newer scenes are no longer generic, so every girl has her own renders and story line, which increases the render and scene costs by a very big margin.
A very simple thing at the spa could be stitting with the girl on the waiting sofa and just talk. But this would mean all the problems with three outfits per girl and in addition I don't think it would make anyone very happy to have that.
It would be just another (more or less boring) chat/talk scene.
What are you doing here? I'm waiting for my massage. Oh yes, me too. I hope I get Renée, she's really good at it.
Bla bla...
I'm pretty sure no one is looking forward to a more or less empty scene just for the sake of having something to do at the spa.
So as DuckSwim said, until someone can come up with a really good idea that would be worth the render effort, my time working on the game is better spend doing other things.
EDIT:
On another note, opposed to the opinion in the last review, the game is most definitely not a vehicle to showcase renders.
It is quite the other way round. The renders are necessary to drive the game and to have a game at all.
If I just wanted to render, I'd go back to DA and publish renders. I'm rendering a lot of images necessary for the game, that are not really fun to do or make me happy. But they are necessary for the game, so they are done too. As long as the balance is okay, I don't have a problem with that.
From my origin I'm still a tech guy who loves to code, learn and find out new things. I'm not an artist by heart.