Most of the Americans on AW in London are extremely over priced.
Most Americans have seen in UK actually been less than when I was in US. No attraction for them to come to London to charge less if they can earn more back in USA or at least comparable as they will have equally high costs for living for their bills when they go back home to any major city in the states.
Similar trend from any girl coming yo London from any first world developed country with similar economy or better economy than UK - save with a Swiss girl, Canadian, Australian, Scandinavian, Asian - Japanese etc.
That is why the cheap market exists with all those east European girls or other poor country girls coming to London as the money still ok for them when they send back home.