you should link for the sake of visitors, not
PR. If the link will bring you a potential customers or somebody who will subscribe to your newsletter or do whatever your goal is on your site, than that's a good link.
to answer your question though,
PR is not calculated with an average but an addition.
If
PR is apples and a site (PR5) sends you 2,000 apples, my site (PR0) might send you 10 apples.
This is very simplified of course but you get the idea.