For one of my current clients, I still bill only the time I actually work. It's a long-term project with no explicit end-date. I'm trying to get out of, I want to do shorter-term projects now, but while I'm here, they've hired me for the fact that I can crush out code and make things better than their own employees. They didn't buy a specific thing, they buy my expertise.

It means I'm not making as much as I could be, but it's not their fault I agreed to the arrangement 3 years ago.