This is how it was explained to me by a driver. I travel 7 stops, and 9/10 drivers charge £1.70 but once or twice they've asked where I was going, I told them and they said that's £2.50. In essence 80p to go the two extra stops.
However as I say it's very rare for a driver to actually give a flying shite where you're going. Depends whether they have a stick up their arse.