This must certainly be the question most-often asked by students thinking about careers in law teaching (well, after the question, "How do I get in to law teaching?" but I've answered that question at length elsewhere). As it happens, there are three public sources of information on the subject, which don't give an exhaustive answer, but certainly are highly informative.
First, a group called the Society of American Law Teachers, collects data each year on median salaries at each professorial rank from schools willing to report it; the most recent data is here. (Keep in mind that most, but not all, law schools promote straight from the untenured ranks to "Professor"; indeed, some law schools don't even have an Assistant Professor rank--untenured faculty are "Associate Professors.") One difficulty with the data is that it appears that some schools are reporting the 12-month salary, while others are reporting the 9-month salary figures. In law schools, "summer money" (which would be reflected in a 12-month figure) is an important part of the compensation package, accounting for anywhere from 5% to 25% of overall compensation. "Summer money" is usually awarded competitively, to support active researchers, but practices vary from school to school, and at some schools the money is basically guaranteed. It's not always clear, however, from the SALT data whether that money is included or excluded in the figures reported for each school.
Second, the University of Virginia's student newspaper publishes each year the salaries of all faculty at UVA, including law faculty. The most recent data is here. Bear in mind that UVA is at the very high end for law school salaries (it is part of how they've maintained a top ten law school in an isolated location at a research university that is overall barely in the top 25, if that).
There is one peculiarity of the UVA salary structure also worth keeping in mind. With a handful of exceptions, most UVA faculty rotate out of endowed positions that supplement their salaries every 4th year. Someone on a "9-month faculty" status is presumptively not currently holding the endowed position, and their salary will have usually decreased from the year before. The very next year, of course, they'll rotate back in and their salary will go way up. Even allowing for that, UVA salaries are extremely high, and very market-driven, rather than seniority-driven (witness the salary for G. Triantis, who was lured back to UVA after defecting to the University of Chicago--he is quite a bit younger than, e.g., Glen Robinson, or Kenneth Abraham, or Robert Scott--not to mention Walker, White, BeVier, and many others.)
Finally, the Chronicle of Higher Education each year collects the tax returns of private universities, which, by law, must list the five highest-paid faculty, plus the salary of the President. Law professors will sometimes show up on these lists--except, of course, at universities with medical schools (medical faculty always dominate the top five) or with really deserving coaches of major sports teams. This data is on the CHE site, but requires a password to access, so I'm posting here some highlights for the 2000-01 fiscal year, the most recent available:
At Harvard University, we can glean from the data that no law faculty member, including the Dean, earned more than $280,000.
At Boston College, the Dean John Garvey (a very distinguished constitutional law scholar, who moved to BC from Notre Dame) earned $409,012, while the highest paid regular faculty member was Daniel Coquilette at $290,615.
At Hofstra University (on Long Island), Lawrence Kessler (whom I've never heard of, I must confess) made $229,273, while Hofstra's best-known faculty member, Monroe Freedman (a leading scholar of legal ethics), earned $213,976.
At the Illinois Institute of Technology's Chicago-Kent College of Law, A. Dan Tarlock, a leading figure in environmental law and probably Kent's most eminent professor, earned $192,266.
At St. John's University, a very regional law school without a substantial scholarly profile (other than the woefully underemployed Brian Tamanaha), the Dean Joseph Bellacosa made an eye-popping $374,232.
At University of San Diego, their first-rate Dean (who has already transformed the institution) Dan Rodriguez, who came from Berkeley's law faculty, earned $210,196, while Michael Moore, a leading legal philosopher and criminal law theorist (who is now at the University of Illinois, but came to USD from Penn), earned $217,642.
Meanwhile, at Washington & Lee University, a good regional law school in Virginia, the Dean, David Partlett (who came from Vanderbilt), earned $295,640, while two leading members of the faculty (the two leading faculty? perhaps) earned $176,585 (David Millon, in corporate law) and $175,500 (Doug Rendleman, remedies).
And finally, having nothing to do with law, in 1999-2000, the great psychologist Daniel Kahneman at Princeton, who won the Pseudo-Nobel Prize in Economics last year, was among the five highest paid faculty at $270,480...but the next year, he dropped off the top five list!
Bear in mind, of course, that these are simply salary figures, and don't reflect, e.g., what law faculty earn from consulting or sales of textbooks and so on. In some cases, faculty earn quite a lot! When I first got to the University of Texas in 1995, faculty would occasionally joke that the salary the University paid our late great colleague Charles Alan Wright--senior author of the preeminent treatise on federal civil procedure, a treatise so influential and authoritative that Supreme Court Justice Ginsburg called Wright a "colossus" atop the legal profession--was what he used "to pay the taxes on his real income." There aren't, to be sure, that many law professors who command more than $1,000 per hour for their time (but there are more than you might imagine!), but there are hundreds, and perhaps thousands, who have more $400/hour consulting work than they can accept.
Consulting opportunities in jurisprudence are, alas, a bit limited...