I think the best freemium model which with my previous comment would be to pick about 10 "dynamic" topics, along the lines of say todays current google trends h

I think the best freemium model which with my previous comment would be to pick about 10 "dynamic" topics, along the lines of say todays current google trends http://www.google.com/trends/hottrends http://www.google.com/trends/hottrends which would give people enough of an indication of the utility of your site.