The Canadian Association of Law Libraries and the Toronto Association of Law Libraries conducted a joint salary survey in 2018. The results are available on the CALL/ACBD website under Publications or from this link.
Earlier editions of a CALL/ACBD salary survey are also published and available.
My opinion: anytime you hire a law librarian you are getting plenty of expertise (most respondents had a ton of education), personal investment (lots of respondents with a ton of time with their current employer), and law librarians when compared to say, a first-year associate, don’t cost much at all (2013 average law firm librarian salary: $66,590; 2018 average law firm librarian salary: $74,914). Heck, for that much, hire two law librarians.
Note: In this post, I am using ton in a non-statistically probable manner.