Do an Internet search on “measuring PR” or the “value of PR,” and you’ll find any number of blog posts and articles that purport to tell you how to do this.
Unfortunately, anything that could have been useful about them usually starts and ends with the title (this post from Chris Penn is a notable exception). The posts will tell you why AVEs suck (d’oh). They’ll tell you all about clipping services (double d’oh). They’ll talk knowingly about how “strategic, proactive PR” can affect a business’ bottom line.
They often stop short of giving you anything you can actually use. It drives me crazy.
Measuring PR is complicated
The problem with these posts is that they’re trying to address a nuanced subject head-on (also that they are often written by people who don’t actually understand what PR is, or how to approach it strategically).