The National Law Journal reports that, according to Hofstra University School of Law professor Richard Neumann, “a law review article written by a tenured professor at a top-flight law school” is “in the neighborhood of $100,000”:

His estimate factors in the salary and benefits for a tenured professor at a high-paying school who spends between 30% and 50% of his or her time on scholarship and publishes one article per year.

It also takes into account possible research grants, which many schools offer professors to help fund their scholarly work, and the costs for research assistants.

ABA Journal has additional coverage here.

It would be interesting to see cost estimates of academic writing in other disciplines to see how the law compares.  I’m guessing that law may be on the high end insofar as law professor salaries are generally higher than most other academics.