If it wqas only one window then I wouldn't charge anymore, I would debate about charging for 2 extra windows too, particularly if it made next to no difference to the time taken, but four 2nd floor windows would see me putting up the price by a fiver, these are not windows that are easy to get at off a ladder, and the higher you go, the harder it gets with a pole too.
Price the work up as if you were doing it traditional, otherwise you are going to end up doing more work for less money.
WFP may be faster, but if you cut your price accordingly, where the hell is the financial benifit!!
Plus brushes and jets wear out, poles have to be replaced, resin must be bought, R/O's are not cheap to replace, pre-filters need changing regularly, and of course you still have to cover your initial investment.
Your general prices shoud go up, and never down.
If you price up...say, a large house with georgian windows, and don't price it up as if you were going to do it trad, you are going to be in a quandary if you then, because of severe weather, have to clean this account the trad way.
There is a grave danger of giving prices that are too low because of the speed at which WFP can be done.
It isn't as if you don't still need all your trad gear remember, so the running costs of your WFP are over and above your normal running cost, they of course may be reduced, but they are still there.
This particularly applies to larger accounts, with a small semi the time difference isn't great enough to make a difference.
So if you are doing extra work, charge for it!!
Don't forget, your customers will already be getting mor bang for their bucks by having all the frames washed down, and I also wash all of the doors down as I go too!
you don't get more for less in my world
Ian