Difference of professor salaries between different department in the United States
Here's one that's readily accessible, though the full data isn't public (but can be obtained).   Here's another one in a much more concise format, though a bit older.

At least for public universities, it's not too hard to do such a survey since our salaries are public record.  I haven't actually checked but I would imagine that the groups that have done these surveys keep them updated every so many years so you can use the ones I linked to to find more.