I have taken tons of photos over the years and some video. My experience is that the more experienced the girl and, obviously, porn chicks are more likely to allow it but as you point out, for an additional donation. And as my mother used to say, no harm in asking! If you ask and she says no, you're in no worse shape than if you hadn't asked!
that should be worked out ahead of time. Many providers do not want their faces made public, for many reasons. I'd bet money, most would say 'hell no' if you sprang that on them when you got there for your date.