bwnbwn - 5:00 pm on Mar 20, 2012 (gmt 0)

So my question is how much do the broken external links hurt their rankings.

Broken links have been part of the algo for many years. It speaks about the site upkeep and or lack of. Some htm sites have grown so big they are impossible to fix or a better word lack of time to fix. These sites may still be adding new content thus the broken links won't hurt as much probably because of their new more modern linking profile. The sites that are just forgotten probably don't fair as well. They still will rank in the long tail for now but lost the key traffic terms that they might have commanded in the beginning years as lady time moves on so the sites do as well down in the abyss of link rott.