It all depends on how much traffic your site receives. You need traffic to make money, and to get traffic you need to rank well in search engines.
well it depends on which niche it is..but may be $5 to $15 provided u can find right buyer. Although there are good chances that your PR may get reset to 0 because of link selling
You should check for sub page page rank as well. When selling sites for PR value people tend to go with site wide links (at least in my experience) they are the best for building pr. Typically a page with a P4 home page will have a few sub pages with pr1-pr2 which will also increase the value of the link. I would say anywhere between 5$/mo and 50$/mo (per link) for the site depending.
You could get 5$ per link easy, depending on other various things such as total indexed pages, domain age, and the number of other outbound links. Just don't sell like 20 links at once because they'll all become worthless.
A friend of mine runs a PR3 site, sells 8 links on it; pulls in about $200 a month from it. I don't know the specifics of how he does it, but I've seen his paypal account and he ain't lying. Of course he is also well aware that this might not last forever and lives in constant fear of the big G.
Google. He's living in constant fear that they'll find out he's selling links and shut down the PR on the site.
If you are selling the link to someone who is seeking for link/SEO juice, then traffic is not a big concern anymore.