Assumption is the mother of all fuck ups. It is true to say there is no direct correlation between price and quality although I think it does depend exactly what you want from a punt.
I would say some punters want little more than a warm body attached to some wet orifices and genuinely don't see any reason to pay more for one body/orifice vs another. Ok slight exaggeration.
Other punters want some very specific services ranging from dfk to owo to cim to a levels, bdsm or role play scenarios and most girls who offer these charge a premium for some or all of these services - either included in their standard rate or as extras.
If you fall in the former camp you can pay under £100ph. For the latter up to £200ph seems about the general limit above which you appear to get literally nothing more for your money.
The £200+ ph escorts are offering no additional services nor do they generally appear to be prettier or have more perfect bodies. None of them have gold plated vaginas.
In all three price ranges you can find escorts who deliver a good service and those who deliver a bad service. For me personally I have never seen any well reviewed girl that offers the services you want for less than £120ph and I see no reason to go above £200ph.
The best way to find a good punt is simply read the reviews on this site, find a girl who is well reviewed that you fancy the look of and book her. It's not rocket science