How significantly does the salary amount for tenured faculty impact the quality of a deparment?
There are several main financial factors faculty candidates must consider when weighing offers:


Salary and resultant standard of living
Overall compensation package (health, retirement and pension, other benefits)
Institutional support (resources, personnel, infrastructure)


The first two are with respect to the financial resources committed directly to the faculty member; the last is the amount of resources committed to the professor's working group. It is difficult to say how much the different factors are weighted, as this is highly subjective and personal.

However, I would caution against considering absolute salaries as a metric; salaries always have to be considered relative to the cost of living in one's environment. (Would you rather have $50,000 in New York City or $40,000 in Akron?)