I almost always charge by the project.  On the very, very rare occasion (usually for specialized and indefinite work) I’ll charge by the hour.  The cost here varies based on the nature of the job.  Since this only happens occasionally, the nature of the work has to be outside of the normal scope of work that I take on.  Usually, this involves some kind of consulting or training work and doesn’t involve writing in any way. I never charge per word.