Beat me to it, Steve.
I tell them the same. They're trying to describe their windows, I'm trying to imagine these windows, and if I give a price over the phone I'll normally be wildly under, or wildly over, what the cost for the job should be. If I'm under on the phone they'll think I'm trying to rip them off once I'm there, if I'm over they won't even entertain it before I get there.
I tell them that we're not the cheapest in town or we wouldn't survive, we're not the dearest, or we wouldn't get enough work to stay in business, but that we've got customers who've been with us for twenty years so we must be doing something right. That usually works.