There are so many variables it's impossible to do this. Does the link use anchor text that is a very valuable keyphrase for you? Or is it just your domain name linked? Is it from a site that is in your field? Is it from a site with an .org or .edu tld, or from another authority site such as wikipedia? Is it from their front page or buried five levels deep? Is the accompanying article flattering to you or is it a
pr nightmare? Does it bring you traffic that converts, or traffic that just wastes your bandwidth? Is it from a good web neighborhood or a bad web neighborhood?
And that's just off the top of my head. I'm sure others can make that list of variables much, much longer.
And how should you rate each variable? None of us knows.
I think there are many more important things to focus on than putting a dollar value on a link. For instance, getting lots more of them . . .