OK I will throw my prospective in as well, hopefully wont make for too heavy reading... lol
I view it like this, you as the "production company" know what your budget is, your in business so it's your job to minimise costs (and obviously maximise income)
I always have a figure in mind, and hope that the girl has a rate in mind too... then it's just a matter if the two meet. Yes I negotiate on price, I dont have a endless bucket of money, so when working to a budget the more I can cut costs the better... of course without losing quality...
There are definetly other factors that have been mentioned... firstly who the girl is, some girls can demand a higher fee... however I have to say I have been lucky to work with some of the best, and all worked to my budget.
I also never book per hour, or per scene... I book a girl for a full day (8 hours, which includes breaks of course) I tell the girl what we're looking to achieve as in the number of scenes... For the harder stuff it's usually around 8 scenes in a day, so 8 picture sets and 8 videos.
Levels also come into it... especially if your expecting extreame stuff... Big toys or anal probably warrent a extra few quid !
If your looking for ball park figures... for hard girl solo and to be honest hard girl girl we pay between ?300-?350 a day... so Jasons ?50 a hour aint too far off I guess
Finding talent can be hard... a few of the "perfect girls" for your stuff have recently retired
But there are defo some out there, with your background I would strongly recommend talking to Samantha Benthly for one...
Good Luck