With agency girls, my experience shows that £150 girls are generally poor and £180 - £200 girls much better so in this case there is a definite correlation between price and performance. Saying that the £250/£300 or even £600 agency girls are unlikely to be any better than a £180/£200 one.
My experience has been the same but specifically with Hot Collection and their Polish girls. Their £180-200 girls are usually very good, while their £200 Russian girls are usually absolute stunners but more hit and miss when it comes to service.
With Maxes, the correlation rule applies in that £250+ girls are on average better than the £200 girls. Reason being that it's one agency which moves girls into the higher price category only after they have established their popularity and reputation to some extent and it's not a case of only the girl setting her prices)
With agencies like Hamiltons, I have found price tends to be correlated more to looks than service. They are very good with the whole booking process and helpful trying to find you the right 'fit' when you try to pick a girl, and the higher priced girls tend to look very good, but the service can be very hit and miss.
Bijoux similar, except that I can't recall girls there below £250, while Hamiltons have a larger selection in the £200-250 range.
Allure is one agency which has higher prices (no one below £300) but my limited experience suggests that they are quite focused on girls who both look good and provide a good service.
So bottom line -
At £150, you are definitely better off with well-reviewed independents - plenty of very good girls at that range. Can't think of any cracking agency girls at that price (used to be the case until a few years ago, not any more)
At £200, you are getting into reliable EE (Polish) territory but read reviews to separate wheat from chaff.
Above that, all bets are off - only reviews can narrow your options down sensibly.
