If your public game is strong, going private has to cover the cost of lost time. And also how ever long it takes for your room to recover.
There are also models who have a strong private game. And they're making most of their tokens from privates.
From what I've seen, lots new models start off at a min of 500 tokens for a show. No up front tokens. They get an up front agreement on what the show is and how it's going to end. Along with how many tokens your going to spend; so they can pace the show accordingly. Usually the shows starts off with a strip tease, or just going slow, basically padding the first couple of minutes. Bailing screws the viewer over more then the model.
I can't speak for any one else, but i'm always paying attention to how a model delvers from their tip menu. If they're responsive, making a show for their tips, that's usually a good sign for privates. If a model is forgetting people's tips, slow to respond to tips, trying to skip tips, having to be reminded that you tipped, then i'm worried they'll be doing the same thing in private. And probable won't ask about privates.