Ours asked how many windows and quoted us 16 quid.
Another in our old house said approx 15 to 20. Agreed to do the first one for 15 and asked for 18 For subsequent ones which I thought was fair.
Do you have an average-sized house? How frequently does s/he come? How long do they take? Do you live in 'the posh bit' of Knightsbridge?!
I'm not denigrating the job that window cleaners do, but every one that I've seen only seems to spend a couple of minutes (if that) at each window, with a squeegee and a bucket of soapy water. Maybe 20 minutes at each house? Other than the pressurised pole thing to do upstairs without a ladder, what do they do that you couldn't easily do yourself?
From what I read on MN, house cleaners don't charge that much per hour, and their work involves scrubbing, contending with dirty toilets, working around furniture and possibly clutter etc. Why does a window cleaner potentially earn 4-5 times more than a cleaner? Is it just because most cleaners are female and most window cleaners are male, so the latter deem themselves worthy of much more?
Each to their own, but considering that most people don't feel the need to have the budget to employ window cleaners, that sounds like a huge amount to pay.
Regardless of price, though, this sounds fishy as anything. 8pm is late to expect to come to a stranger's house, but even if it was 2pm, as PPs have said, why ever does he need to look inside? It's much easier to survey the windows by walking around outside than from inside, having to open and close doors and go from room to room.
At best, he's hoping to put a lot of pressure on you to enable him to secure the business and make you feel unable to say No or Maybe, rather than looking around outside, leaving you with a quote and saying "Just give me a call if you want to go ahead" (maybe he's asking for an unusually high price). At worst, well....