What's the ROI of a blog?

What's the ROI of a blog?: "We had an interesting conversation with a prospect for our new CGM Dashboard software. As usual we explained that it was specifically geared for consumer generated media parameters such as comments, trackbacks, nature of comment, type of comment etc. there was a lot of head nodding and hmmming and then at the end the PR guys said 'but what’s the bottom line? When I go into my boss he expects to see a dollar sign like an ad value equivalency or something. Don’t you have that formula? ' "