I run a free forum host which has 4k+ forums and has a lot of traffic.
Stats:
URL:TalkThis.com
PR5
14k+ uniques/day
100,000+ pageviews a day
Your link will appear on over 150k pages
Alexa: 50,299
For a sitewide link we charge $65/month through textlinkads.com (http://www.text-link-ads.com/packageDetail.php?packageID=45765)
As you can see we have 5/6 spaces sold.