Certainly a job could spiral out of control and end up loosing me money (relatively speeking)
While I agree with your quoting practices, you must have some idea of what your time is worth to be able to say this. ;)
Having an hourly wage worked out with your client is great for unexpected and unrelated work such as fixing the database that they've screwed up, yelling at their hosting company for them, or letting them talk your ear off on the phone about the feasability of projects that they'll never start. Some clients are great at occupying huge amounts of your time with project-unrelated tasks.
Unfortunately, some people balk at high hourly rates even though the currently accepted wisdom is that a great programmer yields ten times the productivity of an average programmer.
What is the going rate for programming services per hour, for contract work?
In my experience, $40CDN/hr for web development contract programming.
This is an area where I have special expertise, and am probably the best person anywhere for the job.
Now you're bringing supply and demand into the equation. The answer is somewhere between "what you feel your time is worth" and "as much as you can get away with," depending on how much trouble you have sleeping at night.
In my experience, $100USD/hr for web development contract programming.