I think that there are a lot of 'special deals' being offered, to make punters think that they are getting value for money. A local girl I know will always offer a 'reduced rate' because it's a Tuesday, because it's a month with an 'R' in it etc.
The problem is that if your headline ask price is £100 an hour, you're never going to get some mug to pay you £180.
The main thing, though, is that a high price sends a signal that the girl is high class. It's hard to judge the quality of the service you're going to get (although UK Punting and AW feedback is gradually changing that) so a high price means that the girl moves into the 'reassuringly expensive' category.
It's like London high-end property. Nowadays a huge proportion of properties in central London which were advertised for sale end up being taken off the market (without being sold). The sellers can't afford to drop the price, because this just will signal that they are a 'motivated seller' and they'll find that any buying interest they get will try to wrestle down the price further.
Having said all that, with the flood of new supply to the market, I think that rates have to fall further. The number of pimped Brazilians and young British mums looking to make their housekeeping go a bit further must be adding to supply faster than demand can keep up. My feeling is that rates have staying about level for years, which means that in real terms they are dropping at a rate of 3-4% annually but even that is not enough IMHO. No wonder the game does not attract girls with sharpest intellects. As a business proposition it sucks, in more ways than one.