That's funny i was asking myself the same question a few days ago. I mean, is there a cost associated with having a girl in the roster? I'm not sure there is.
There's the cost of the pictures but it shouldn't be too high, maybe the cost of renting the incalls but my guess is they're renting them anyway.
On the other hand having a lot of girls on the website gives some credibility to the agency even when they're only working a few days a month.
I'm thinking the main reason to let a girl go would be when she when she can damage an agency reputation but besides that i'm not sure there's a financial reason to do it, but maybe i'm missing something