I use ungers equipment and liquid to do my downstairs outside windows. It involves sloshing liquid onto the windows and it runs down, messy. Upstairs have never been cleaned, they clean themselves when it rains
I also do my indoors windows, it is more difficult and takes longer. I use a method spray and 2 microfibre cloths, needs to be done on a dull day or lots of streaks appear in the sun
If I needed someone else to do inside, I would ask a cleaner or would just use a spray with diluted ungers and a karcher. I do my insides in stages and I do need to use steps. A small set is all I need. The karcher has a bit of height and makes it easier to do the top or ungers squeegee on a short pole for the top and catch the drips with a karcher or cloth
I could see up to £100 for inside, a fair price. I have similar number of windows, 16 windows and 2 double french doors. That is equivalent to 20 windows at a fiver each on average