When I first started window cleaning, I did a little sub-contract work. I was asked to quote and I provided a fair price. The contractor then sucked through his teeth and told me the maximum price he would pay; and because I had loads of spare capacity; I took the under-priced work on. As my spare-capicity decreased; I dropped the sub-contract work.
Remember:
The contractor wants to make a profit out of you.
He may also be not that quick at paying.
My advice would be, if you don't really need the work; quote much higher than you'd expect to earn cleaning residential for the estimated time it will take.
However, if you need the work; quote high again; but be prepared to accept a lower payment and a delay in getting your cash.
At the end of the day; there's not much difference from cleaning the windows on a house, or the windows on a shop (even if it is a big one).
I hope some of this helps.