I think this is about the cheapest way to do it for doing first floor (tops) only and some ground floor work too (leaded/Georgian windows):
(All prices approximate)
Shurflo Back Pack: £215.00 (Inc VAT and delivery)
Pole: £80.00 (Get a good one - Ionics 18 foot is good)
Brush Head: £30.00
Water barrels x 6 £35.00 (very approx)
Hose: £30.00
RO/DI 50 Gall per day £70.00
Some Hoselock fittings £10.00
Water Butt £ 40.00
Total £520.00
There's probably one or two things I've either forgotten or priced wrongly; so add another £50.
So you could be up and running with a WFP system for around £570.00.
It's worth it for numerous reasons.
Also don't worry about what your customers may think; apart from 'great'. Drop the strange ones; which I haven't had to yet, if at all.
I still use ladders though, if I think it's going to be quicker or for access reasons (flat roofs and the like).
I picked up two new customers today; both times it was raining; and I was using a WFP.
Some of my current customers don't even bat an eye-lid at me using a WFP; until I give them my 'information 'essay'' and start explaining. One customer has even told me 'enough!', as long as my windows are clean; I don't care how you do it.
That's good enough for me.