As said before John, if you put up prices that much, i think you will loose lots of customers, you could do them WFP, ad as said before, do them in a lot quicker time, so you can do £9 of work in the time of a £6 traditional time, but again you might loose a lot of customers who dont like the windows left wet.
I know of a local window cleaner who bought similar work, converted to WFP, put prices up over time, now he gets good , good rates , and does £5k a month
idealrob