They like to think that they are the world's best negotiators. The truth is, nasty customers like to give that perception, although it is not at all reality. They will bend when shown that benefits far outweigh the price and that service is a necessity rather than an option. They will pay your price if their need is great enough. It is up to you to develop that need.
All nasty customers have a soft spot somewhere.
You have to find it. Get them to smile. Get them to laugh. Say nice things about their mother. Find out if they like flowers.