Negotiation like this is not simple. If you are running your own business I would recommend some negotiation seminars. They will pay off in the long run. I have taken courses at Karass with great success. In your situation it is all about information. What does the client know? Do they know that they are the only customer of yours? Are you willing to walk away? Does your contract with them include terms for renegotiation? I do consultant work from time to time, and when I am busy I quote outrageous rates, sometimes I get the work, sometimes I don’t. I would say you should approach your client and say something like “My time is limited, and I have other customers who are willing to pay me much more to do similar work. I have to make a business decision to keep you on as a client. But only if I can market adjust the rates.”