I believe from what I've been told that established low-ratio parties pay around £60-70ph for quality girls, but they offer around 6hrs work a day for a couple of days a week with all the punter comms, venue costs & backroom stuff sorted, so the girls see it as a good way to earn a guaranteed £400-ish per day that might see their weekly living expenses covered.
My guess is that if you would want to arrange an ad hoc party with a bunch of well-reviewed prossies, you would need to be offering more than their current 3 hr rate to tempt them, esp if they were being expected to service a high ratio of punters.
Some party providers seem to have this issue, the girls want so much per party that they have to charge a fortune to cover costs and make some profit. Once you've made a rod for your back it's hard to renegotiate.