I've only been cleaning for 3 months but have had a lot of help from other people on this site - so I hope this helps.
I found out what other cleaners were charging for particular houses and then devised a pricing formula based on the number of first floor and ground floor windows - e.g. in my case it was 50p for each first floor window (£1.00 if it involved 2 ladder climbs) and 25p for each ground floor window (50p if it was a large picture window).
Because I always wipe/clean the frame as well, I feel that I have undercharged on many of my original houses. However, I did get plenty of compliments and some recommends.
I now add 20% to any calculated price to take account of the extra work.
If the frames are really filthy and/or the customer has not had a window cleaner for some time, you should charge extra for the first clean.
I sometimes think I should be charging more. I haven't yet decided whether I'm doing too good a job or whether I'm just too damn slow.
Good luck,
Mike