Interesting point, and one i'd not considered.
I have seen one SP 3 times now, and come to think of it, certainly for the last 2 visits, it was money afterwards, although i had it ready at the start of the second visit, to which she simply said "oh just leave that on the side i'll get it later" and the 3rd time it wasn't mentioned until the end of the session, and i brought it up.
The first visit i can't remember, i would suspect it was at the start, as i would expect, especially on a first visit.
But yes, all 3 visits were good and the effort put in my the SP was perfect.