In recent months google has taken a dim view of paid links in blogs etc. They prefer them to be nofollowed and show that its an advertorial. That being said lets assume your keeping this under the radar and are not going to have any issues with negative SEO.
In my opinion it's hard to put a price on it there's diminishing returns as the volume increases however the exact numbers etc i'm not aware of. I would think if you have a link from them every few months it would show they like you but if you have too many to quick and the content is not relevant I would think the benefits would be less. From a PR and Traffic point of view its good publicity so i would price it based on the potential customer impact and slightly ignore the SEO side of things after the first half dozen or so.