Check if a backlink has any benefit for your page, and how many such links you need in order to increase your page's PR by 1 point.
The primary target of any link exchange deal is to get higher PR.
To see whether the link exchange deal is fair and profitable for your site it appears to be quite essential to check the usefulness of back link i.e. to find out how many suchlike pages should link back to your site/page so that its PR increases by 1 PR point in future.
Please remember that a link with www and a link without www are two different links for google.com