This is interesting (albeit a bit shilly): Careerbuilder's The Hiring Site blog is pimping their recruitment metrics, which you, too, can have access to if you use their services!

Their Sourcing Effectiveness Formula is an interesting way to look at recruiting ROI...ignore the advertisement if you must.

"Say a job seeker is conducting a regular ol' organic job search... The Sourcing Effectiveness Formula measures the searches where a company's job listing appears, the number of times candidates click that listing from the search results, and then the number of times those candidates that viewed the opportunity click apply. From there, it counts the actual number of applications that result. Then Personified looks at how this search/application ratio compares to others in your industry."

Then the Careerbuilder people break down what each of these mean. If your ad isn't coming up in search as much as others in your industry, the ad might be poorly written. If your ad isn't clicked as much after it actually comes up in search results, your employment brand probably stinks. The number of people who actually decide to apply for the job usually measures the effectiveness of the job listing itself...and the number of people who finish the application corresponds inversely to how hard the dang thing is to fill out.

Careerbuilder says they can track all this for you, the job poster (of course) but there are other applicant tracking systems out there, too..