Of course you can look at Google backlinks, but that is partial.
You can look at ATW links, and that may be more helpful.
You can look at Alexa traffic rank, but that is almost useless.
It seems to me that a value per link may be the best way, but of course not all links are created equally.
Anyone have a good formula they use?
That percentage takes into effect the fact that the only work I need to do is push them to the buyer, as opposed to building content, promoting it, and so on.
So I guess 'my' (Again, the imparitive word being MY) answer to your question is: It is only worth what you can make from it, and the 'for sale' percentage depends on how much work you want to do.
Just my 2 cents...
DS