If I ever get a potential customer haggling over the price, I tell them what they get for the price, ie frames, sills and doors (as well as the windows obviously).
9 times out of 10 the job is mine.
The last time I didn't get the job on price was when a chap said 'I don't want to pay that much', which was £20 for a 3 bed semi with a large conservatory. I said 'OK, I'll leave it with you' and thanked him for his enquiry. I hope he doesn't ask again because it seems to me that anyone who has this attitude will drop you the minute someone cheaper comes along.
John.