If your work on the glass is good and are people friendly then be confident in the service you offer and in yourself. Reflect this in your prices.
Did one today, pulled up to clean a new customers windows (3rd clean) and noticed most of neighbours cars in drive and thought whilst here, wont hurt to ask. So knocked and explained and gave a price of 15 for 4 wkly cleaning. Big chap who looks after himself, "tell you what make if 15 for 8wkly clean" he says.
"Call it 20 for 8wkly and youve got me booked!"
I now have a new customer. (Now i knew before the lad i have taken over used to clean this house before he messed things up on his round and he only charged £9 a clean. This was about a year ago)
Not doing all this waffle for it to be contested, but just top tip, be confident when going in for a price