Hourly rates for industry consulting? I have a friend who is considering a part time (~10hr/week) consulting gig with a large, well-established software company, doing a mixture of research and coding.  However, there appears to be little quantitative data on past consulting rates for academics, i.e., professors, postdocs, and PhD students whose primary career is not consulting.

Q: How does an academic determine a fair rate for consulting?

...Assuming that a consulting gig is actually compatible with their academic contract! I do of course realize that in many circumstances external employment is prohibited by a contract with the university.  I also realize that consulting rates are largely a function of 1. perceived expertise and 2. negotiation skill.  But a key component of successful negotiation is an objective view of the facts—in this case, some kind of upper and lower bound on reasonable wages.
As Fomite correctly stated, it is going to vary a lot by field and also individual circumstances.

In my experience, if the consulting work is roughly equivalent to what a non-academic in industry might be able to perform, then the rates will roughly be equivalent, too.  This happens a lot in the engineering disciplines, and especially often in computer science (i.e., many computer science professors can moonlight as software engineers).  In such disciplines, I have found that companies in industry often classify their engineers into five levels.  These go by different names at different companies, but they usually are along the lines of: associate, full engineer, senior, lead, and principal.  Associate is the level a freshly graduated undergrad would start at. Full engineer usually implies three to five years experience and/or a master's degree.  That is likely the level equivalent to a Ph.D. student.  A fresh Ph.D. just having defended would usually start at such a company at the senior level, so it is roughly equivalent to post-doc and junior faculty.  The lead level is usually achieved 7+ years after that, so it is roughly equivalent to tenured faculty.  Likewise, principal is equivalent to full professors.

Once you have an equivalent job title in mind, you can use websites like glassdoor to search for average salaries for that job title (e.g., "Senior Software Engineer", "Principal Electrical Engineer", etc.).  I find that typical hourly rates are in the range of 0.1% to 0.2% of the yearly salary of an equivalent professional, but once again this may vary by profession; my experience is in the world of software.

I have found that some companies will allow a slightly higher-end consulting rate if you can demonstrate that your abilities are above and beyond those of a non-academic alternative.  This is especially true if the nature of the consulting work is research-oriented and/or directly related to your research, or if you can reasonably argue that you could complete the task faster than a non-Ph.D or a full-time employee.

You can usually charge a slightly higher rate for short-term work.

Many academics I know often do consulting in the form of acting as expert witnesses at trial.  Rates for that type of work seem to be more standard across disciplines, and is more of a function of how unique and qualified you are for the job.  Prep work for trial is usually charged at a standard consulting rate, but then days at trial can be charged as much as two to four times that rate.