Hi
I charge £4 per roofing segment, with a minimum charge of £50. (Can take anything from 30 minutes to an hour for normal sized, lightly soiled roofs.)
I use a mixture of methods dependant upon how bad it is, either waterfed pole or extendable pressure lance with rotating brush on the end (the latter is for badly soiled roofs.).
Would advise you stay well clear of just using a lance as the apex of the roofs are often not sealed and water can get in through the top... often shorting the lights out (learned this one the hard way!!!).
If it is particularly bad, you could charge more even for a small roof... judgment call I'm afraid.
Hope this helps.
Andy