Pure greed.
It's not, it's business. None of them are in it for favours. If you can charge £300 and get away with it you'd be a fool to not to.
I saw her about ten years ago and she certainly was well above average and I think I only paid £160 but I wouldn't pay £300 (I wouldn't pay any girl that amount) but there's plenty who are quite happy to. Good luck to her, its a finite business and I would imagine she's coming to the end of her ability to charge that amount.
There's plenty others out there charging less.