I think this sounds fascinating, I will try and give you my view from the limited experience I have. As someone with a background as a masochist anything that happens in person rather than online will always be attractive.
I'm sure there should be a minimum expected amount, but I feel this would be by negotiation and different to each person. But surely it has got to be worth while attending? Personally I'd always prefer to meet in a place where multiple cash dispensers are available, that way the Dominant has options to visit multiple machines to enhance the money transfer.
Personally I would want to connect with the person who I am paying, so interaction would be essential, before during and after. But how she spends that money is not really my concern, so it would not necessarily be a treat. Paying for coffee or dinner or something after the visit to the dispensers would be a good way to further the connection though.