I guess "semi-pro" means they see a lot of men and make a fair amount of their income this way. I guess none of them declare it for tax purposes as they consider it gifts.
They may discriminate as to who they meet, and may also not have a fixed rate and will negotiate it with a minimal expectation. They might also vary the rate dependent on how much they like the man, but either way they expect the minimum payment.
The reality is that if I decide to have, say, a maximum budget of £200 a month to spend on "punting" and the woman needs it to make an income, that's one session per month, (it will also buy me 2 half-hour sessions with Melysa at her current rate). She, however, will need to see a lot more men at that rate to make a living. Could get by on one a day, 5 days a week (especially if she isn't paying tax).
The attraction I find is being more personal, not clock-watched, doing stuff other than sex (in addition to it, not instead of it). Also their appearance on the sites, normally showing their face and fully dressed, is far more attractive to me.