I have personally found that most contributors have a personal investment in their portfolios that goes beyond its pure monetary value. If you were to, for example, mathematically average out how much they are likely to make on the port in the next 5 years and offer them 70% of that, unless it matches their 'perceived' value, they are likely to be unhappy with that offer.

In fact, I get the distinct feeling that most contributors would rather sit on their port earning nothing than sell it for less than their perceived value of it.

That isn't to say contributors are wrong or right; there is a lot of emotional investment in one's media, so it's a tough thing to quantify. However, in answer to your question, you are likely going to have to offer something substantially more than you are willing to pay, or catch a contributor who is so disheartened with the business that they are on their way out.