In the last year I've used them in Amsterdam, Paris, Munich, London(Kensington and Victoria), Glasgow, and NYC that I can recall right off, and it's always been based on supply and demand. I was in London and Paris last month, and in New York City last weekend, and it was still the same (not to say that it may have changed somewhere else). Basically, if they're busy you'll get less time with whatever currency you are using, and if they're not busy you get more time. I don't know how far back this pricing strategy goes, as I've only been using them for the last year or two.