She maybe overpriced to you Mr PepsiCola but to many she will be a VFM and guaranteed top time as called out by the OP, Bluegene and others.
Plus looking at your success rate you maybe best placed to save up and go with someone like CP rather than the cheaper ones you rate as neutral or negative. Be better VFM in the long run me thinks, but hey who knows 
Firstly, I am more inclined to post about poor service providers as a warning to fellow punters. I dont have a record in my head of all the prices charged from my reviews but I could have easily book any of the past reviews for more time, spent more money and reported a poor service...if thats what they intended to provide.
I don't think a guarantee of success is paying more money (or over the odds however one sees it)...a SP provider either does a good service consistantly or they do not.
I also believe there is an element of certain SP IMO literally making up as a high a prices as they think they can 'get away with' (and still have punters ringing) rather them sitting down and working out 'value' and 'rates' 'overheads' and coming up with more considered 'rates.'
(They also probably in some instances, looking around and 'copy' the next SP's prices and may say oh 'they getting away with that, ill try')
This is just using the current SP as hypthetical example: Conservatively this SP prob sees 6-8 punters a day (I know it can be a lot higher with some girls, many are sex addicts to boot, ive encountered and will go past 10) which means a minimum of £2400 - £3200 + extras in a single day.
Shes also probably nearer 35 IMO and still charging that.
Again all this is just my opinion and I thinks shes overcharging.
Edit: Such SP IMO are also either aware the punting landscape is poor in London, or benefiting from it anyway, in that is there is less decent pickings, gents will be more likely to not move past them because their rates are 'over the odds.'
