New Office has a comparison of applicant volume and cost per applicant across different job boards.  The key data is summarized in the table below:

The reported data an analysis overwhelmingly favor Careerbuilder over Monster and Hotjobs. In terms of cost per prospect, Craigslist was the absolute leader, although the analysis soft-pedals this fact.  Craigslist has a broken link to the press release on their job board comparison page, misattributing the information to Yahoo Finance.

I can’t vouch for the methodology or lack of bias of the study, which was reported in a press release.

Both Careerbuilder and New Office are located in Chicago. 

Possible sources of bias include the Chicago locations of the jobs posted in the study and the fact they were all temp positions.

In November 2006, New Office released a very similar press release with essentially the same conclusions; Careerbuilder again enjoyed a 10 to 1 advantage in terms of applicants per ad.

Update: Just to give you an idea of how hard it is to find hard objective comparisons Net-Temps has a similar comparison, except that in their study Yahoo Hotjobs comes out on tops and Net-Temps is number 2.