An interesting WebmasterWorld thread discusses whether a paid run of site link (ROS) is benefital or not. The thread points out some variables that may impact how the search engines will react to the ROS link.
- Age of site
- Size of linking site
- Percentage of links from trustworthy sites
If paid ROS links can hurt a site's search engine ranking, than this could make it very easy for competitors to sabotage others. As mentioned in the thread it seems more likely that the search engines will not penalize but rather discount the link benefit.
Forum discussion at WebmasterWorld.