I'm wondering how others bill a UX service and how it should really be billed. I'm exposed to basing it on hours spent doing the work. However, I don't think that it's the most fit way of doing it because time spent isn't a perfect metric of the quality of work. Some of my better works are done in a short period of time, while it takes me days to finish others.
How do you think should UX work be billed? What are the factors to consider?