You can sell links. I think you can earn 3$ per link. How do you think how much he can sell link monthly?
You can sell the links from your site for example)) ...if you talk about PR There are plenty ways to earn money using sites
If the only quality your site have is the PR, then sell links for profit. You must only sell a couple of them, because Google penalizes links selling!
If it's PR3 with a solid amount of backlinks, you could fetch $3/month on a link, yes - maybe a little more dependant on the quality. Don't sell too many though, makes them less valuable to buyers if there are too many paid outgoing links and Google may penalize you.
it was always mystery to me why someone invest into reaching PR3, while selecting crappy and meaningless $1 .info instead of descent .com
people buy backlinks for the sake of link juice from the high pr domain not for incoming traffic besides alexa rank can be easily inflated with purchased traffic