This is an interesting tool I came across yesterday, i'm not sure on how accurate it is but it's meant to calculate the value of the links on your website (e.g How much you should sell them for) based on PR etc.

http://www.linkvendor.com/seo-tools/linkvalue.html

According to this, I should be selling links on tf for $38.35, i'm only charging $25/month now.....so hurry up you're getting a bargin! lol.